diff --git a/docs/plan/xcui-subplans/Subplan-03_XCUI-Style-Theme-Token.md b/docs/plan/xcui-subplans/Subplan-03_XCUI-Style-Theme-Token.md
new file mode 100644
index 00000000..5217fdaa
--- /dev/null
+++ b/docs/plan/xcui-subplans/Subplan-03_XCUI-Style-Theme-Token.md
@@ -0,0 +1,46 @@
+# Subplan 03：XCUI Style / Theme / Token
+
+目标：
+
+- 建立 XCUI 的样式模型、token 体系与主题覆盖规则。
+- 让控件视觉不再散落在代码硬编码常量里。
+
+负责人边界：
+
+- 负责 `Style` / `Theme` / `Token` 数据模型和解析规则。
+- 不负责 `.xcui` 导入器。
+- 不负责最终 renderer 具体绘制。
+
+建议目录：
+
+- `engine/include/XCEngine/UI/Style/`
+- `engine/src/UI/Style/`
+- `tests` 中 style/theme 测试
+
+前置依赖：
+
+- 依赖 `Subplan 01` 的节点属性注入点。
+
+现在就可以先做的内容：
+
+- 设计 style 层级：默认样式、类型样式、命名样式、局部覆盖
+- 设计 token：颜色、圆角、边距、字号、线宽
+- 设计主题切换与 token 查询接口
+- 写冲突优先级测试
+
+明确不做：
+
+- 不做具体面板视觉重构
+- 不做字体资源导入
+
+交付物：
+
+- 统一样式查询入口
+- `.xctheme` 对应的数据结构
+- 样式优先级与 token 解析测试
+
+验收标准：
+
+- 样式优先级可预测
+- 主题替换不需要改控件逻辑
+- 其他 subplan 能通过统一 API 获取视觉参数
diff --git a/engine/CMakeLists.txt b/engine/CMakeLists.txt
index 439047ae..5ad164bd 100644
--- a/engine/CMakeLists.txt
+++ b/engine/CMakeLists.txt
@@ -413,7 +413,14 @@ add_library(XCEngine STATIC
     ${CMAKE_CURRENT_SOURCE_DIR}/include/XCEngine/UI/Core/UIElementTree.h
     ${CMAKE_CURRENT_SOURCE_DIR}/include/XCEngine/UI/Core/UIContext.h
     ${CMAKE_CURRENT_SOURCE_DIR}/src/UI/Core/UIBuildContext.cpp
-    ${CMAKE_CURRENT_SOURCE_DIR}/src/UI/Core/UIElementTree.cpp
+    /src/UI/Core/UIElementTree.cpp
+    /include/XCEngine/UI/Style/StyleTypes.h
+    /include/XCEngine/UI/Style/Theme.h
+    /include/XCEngine/UI/Style/StyleSet.h
+    /include/XCEngine/UI/Style/StyleResolver.h
+    /src/UI/Style/StyleTypes.cpp
+    /src/UI/Style/Theme.cpp
+    /src/UI/Style/StyleResolver.cpp
 
     # Input
     ${CMAKE_CURRENT_SOURCE_DIR}/include/XCEngine/Input/InputTypes.h
diff --git a/engine/include/XCEngine/UI/Style/StyleResolver.h b/engine/include/XCEngine/UI/Style/StyleResolver.h
new file mode 100644
index 00000000..a29e705c
--- /dev/null
+++ b/engine/include/XCEngine/UI/Style/StyleResolver.h
@@ -0,0 +1,61 @@
+#pragma once
+
+#include "StyleSet.h"
+#include "Theme.h"
+
+#include <map>
+
+namespace XCEngine {
+namespace UI {
+namespace Style {
+
+enum class UIStyleLayer : std::uint8_t {
+    None = 0,
+    Default,
+    Type,
+    Named,
+    Local
+};
+
+struct UIStylePropertyResolution {
+    bool resolved = false;
+    UIStyleLayer layer = UIStyleLayer::None;
+    UIStyleValue value = {};
+};
+
+class UIResolvedStyle {
+public:
+    void SetProperty(UIStylePropertyId propertyId, const UIStylePropertyResolution& resolution) {
+        m_properties[propertyId] = resolution;
+    }
+
+    const UIStylePropertyResolution* FindProperty(UIStylePropertyId propertyId) const {
+        const auto it = m_properties.find(propertyId);
+        return it != m_properties.end() ? &it->second : nullptr;
+    }
+
+    const std::map<UIStylePropertyId, UIStylePropertyResolution>& GetProperties() const {
+        return m_properties;
+    }
+
+private:
+    std::map<UIStylePropertyId, UIStylePropertyResolution> m_properties = {};
+};
+
+struct UIStyleResolveContext {
+    const UITheme* theme = nullptr;
+    const UIStyleSheet* styleSheet = nullptr;
+    UIStyleSelector selector = {};
+    const UIStyleSet* localStyle = nullptr;
+};
+
+UIStylePropertyResolution ResolveStyleProperty(
+    UIStylePropertyId propertyId,
+    const UIStyleResolveContext& context);
+
+UIResolvedStyle ResolveStyle(const UIStyleResolveContext& context);
+const char* ToString(UIStyleLayer layer);
+
+} // namespace Style
+} // namespace UI
+} // namespace XCEngine
diff --git a/engine/include/XCEngine/UI/Style/StyleSet.h b/engine/include/XCEngine/UI/Style/StyleSet.h
new file mode 100644
index 00000000..b9627ee9
--- /dev/null
+++ b/engine/include/XCEngine/UI/Style/StyleSet.h
@@ -0,0 +1,80 @@
+#pragma once
+
+#include "StyleTypes.h"
+
+#include <map>
+#include <string>
+
+namespace XCEngine {
+namespace UI {
+namespace Style {
+
+class UIStyleSet {
+public:
+    void SetProperty(UIStylePropertyId propertyId, const UIStyleValue& value) {
+        m_properties[propertyId] = value;
+    }
+
+    bool RemoveProperty(UIStylePropertyId propertyId) {
+        return m_properties.erase(propertyId) > 0u;
+    }
+
+    bool HasProperty(UIStylePropertyId propertyId) const {
+        return m_properties.find(propertyId) != m_properties.end();
+    }
+
+    const UIStyleValue* FindProperty(UIStylePropertyId propertyId) const {
+        const auto it = m_properties.find(propertyId);
+        return it != m_properties.end() ? &it->second : nullptr;
+    }
+
+    const std::map<UIStylePropertyId, UIStyleValue>& GetProperties() const {
+        return m_properties;
+    }
+
+private:
+    std::map<UIStylePropertyId, UIStyleValue> m_properties = {};
+};
+
+struct UIStyleSelector {
+    std::string typeName;
+    std::string styleName;
+};
+
+class UIStyleSheet {
+public:
+    UIStyleSet& DefaultStyle() {
+        return m_defaultStyle;
+    }
+
+    const UIStyleSet& DefaultStyle() const {
+        return m_defaultStyle;
+    }
+
+    UIStyleSet& GetOrCreateTypeStyle(const std::string& typeName) {
+        return m_typeStyles[typeName];
+    }
+
+    UIStyleSet& GetOrCreateNamedStyle(const std::string& styleName) {
+        return m_namedStyles[styleName];
+    }
+
+    const UIStyleSet* FindTypeStyle(const std::string& typeName) const {
+        const auto it = m_typeStyles.find(typeName);
+        return it != m_typeStyles.end() ? &it->second : nullptr;
+    }
+
+    const UIStyleSet* FindNamedStyle(const std::string& styleName) const {
+        const auto it = m_namedStyles.find(styleName);
+        return it != m_namedStyles.end() ? &it->second : nullptr;
+    }
+
+private:
+    UIStyleSet m_defaultStyle = {};
+    std::map<std::string, UIStyleSet> m_typeStyles = {};
+    std::map<std::string, UIStyleSet> m_namedStyles = {};
+};
+
+} // namespace Style
+} // namespace UI
+} // namespace XCEngine
diff --git a/engine/include/XCEngine/UI/Style/StyleTypes.h b/engine/include/XCEngine/UI/Style/StyleTypes.h
new file mode 100644
index 00000000..a5826f91
--- /dev/null
+++ b/engine/include/XCEngine/UI/Style/StyleTypes.h
@@ -0,0 +1,104 @@
+#pragma once
+
+#include <XCEngine/Core/Math/Color.h>
+#include <XCEngine/UI/Types.h>
+
+#include <cstdint>
+#include <string>
+#include <variant>
+
+namespace XCEngine {
+namespace UI {
+namespace Style {
+
+struct UIThickness {
+    float left = 0.0f;
+    float top = 0.0f;
+    float right = 0.0f;
+    float bottom = 0.0f;
+
+    static UIThickness Uniform(float value);
+    bool IsUniform() const;
+};
+
+struct UICornerRadius {
+    float topLeft = 0.0f;
+    float topRight = 0.0f;
+    float bottomRight = 0.0f;
+    float bottomLeft = 0.0f;
+
+    static UICornerRadius Uniform(float value);
+    bool IsUniform() const;
+};
+
+struct UITokenReference {
+    std::string name;
+
+    bool IsValid() const;
+};
+
+enum class UIStyleValueType : std::uint8_t {
+    None = 0,
+    Float,
+    Color,
+    Thickness,
+    CornerRadius,
+    Size,
+    Point,
+    TokenReference
+};
+
+class UIStyleValue {
+public:
+    using Storage =
+        std::variant<std::monostate, float, Math::Color, UIThickness, UICornerRadius, UISize, UIPoint, UITokenReference>;
+
+    UIStyleValue() = default;
+    explicit UIStyleValue(float value);
+    explicit UIStyleValue(const Math::Color& value);
+    explicit UIStyleValue(const UIThickness& value);
+    explicit UIStyleValue(const UICornerRadius& value);
+    explicit UIStyleValue(const UISize& value);
+    explicit UIStyleValue(const UIPoint& value);
+
+    static UIStyleValue Token(const std::string& tokenName);
+
+    bool IsSet() const;
+    bool IsTokenReference() const;
+    UIStyleValueType GetType() const;
+
+    const float* TryGetFloat() const;
+    const Math::Color* TryGetColor() const;
+    const UIThickness* TryGetThickness() const;
+    const UICornerRadius* TryGetCornerRadius() const;
+    const UISize* TryGetSize() const;
+    const UIPoint* TryGetPoint() const;
+    const UITokenReference* TryGetTokenReference() const;
+
+    bool operator==(const UIStyleValue& other) const;
+    bool operator!=(const UIStyleValue& other) const;
+
+private:
+    explicit UIStyleValue(const UITokenReference& tokenReference);
+
+    Storage m_storage = {};
+};
+
+enum class UIStylePropertyId : std::uint8_t {
+    BackgroundColor = 0,
+    ForegroundColor,
+    BorderColor,
+    BorderWidth,
+    CornerRadius,
+    Padding,
+    Gap,
+    FontSize,
+    LineWidth
+};
+
+UIStyleValueType GetExpectedValueType(UIStylePropertyId propertyId);
+const char* ToString(UIStylePropertyId propertyId);
+
+} // namespace Style
+} // namespace UI
+} // namespace XCEngine
diff --git a/engine/include/XCEngine/UI/Style/Theme.h b/engine/include/XCEngine/UI/Style/Theme.h
new file mode 100644
index 00000000..07248403
--- /dev/null
+++ b/engine/include/XCEngine/UI/Style/Theme.h
@@ -0,0 +1,75 @@
+#pragma once
+
+#include "StyleTypes.h"
+
+#include <map>
+#include <string>
+#include <vector>
+
+namespace XCEngine {
+namespace UI {
+namespace Style {
+
+enum class UITokenResolveStatus : std::uint8_t {
+    Unresolved = 0,
+    Resolved,
+    MissingToken,
+    CycleDetected,
+    TypeMismatch
+};
+
+struct UITokenResolveResult {
+    UITokenResolveStatus status = UITokenResolveStatus::Unresolved;
+    UIStyleValue value = {};
+};
+
+struct UIThemeDefinition {
+    std::string name;
+    std::map<std::string, UIStyleValue> tokens = {};
+
+    void SetToken(const std::string& tokenName, const UIStyleValue& value);
+    const UIStyleValue* FindToken(const std::string& tokenName) const;
+};
+
+enum class UIBuiltinThemeKind : std::uint8_t {
+    NeutralDark = 0,
+    NeutralLight
+};
+
+class UITheme {
+public:
+    UITheme() = default;
+    explicit UITheme(const UIThemeDefinition& definition);
+
+    void SetName(const std::string& name);
+    const std::string& GetName() const;
+
+    void SetParent(const UITheme* parent);
+    const UITheme* GetParent() const;
+
+    void SetToken(const std::string& tokenName, const UIStyleValue& value);
+    bool RemoveToken(const std::string& tokenName);
+    const UIStyleValue* FindToken(const std::string& tokenName) const;
+    const std::map<std::string, UIStyleValue>& GetTokens() const;
+
+    UITokenResolveResult ResolveToken(
+        const std::string& tokenName,
+        UIStyleValueType expectedType = UIStyleValueType::None) const;
+
+private:
+    UITokenResolveResult ResolveTokenRecursive(
+        const std::string& tokenName,
+        UIStyleValueType expectedType,
+        std::vector<std::string>& visiting) const;
+
+    std::string m_name;
+    const UITheme* m_parent = nullptr;
+    std::map<std::string, UIStyleValue> m_tokens = {};
+};
+
+UITheme BuildTheme(const UIThemeDefinition& definition, const UITheme* parent = nullptr);
+UITheme BuildBuiltinTheme(UIBuiltinThemeKind themeKind);
+
+} // namespace Style
+} // namespace UI
+} // namespace XCEngine
diff --git a/engine/src/UI/Style/StyleResolver.cpp b/engine/src/UI/Style/StyleResolver.cpp
new file mode 100644
index 00000000..909c56fc
--- /dev/null
+++ b/engine/src/UI/Style/StyleResolver.cpp
@@ -0,0 +1,139 @@
+#include <XCEngine/UI/Style/StyleResolver.h>
+
+namespace XCEngine {
+namespace UI {
+namespace Style {
+
+namespace {
+
+bool TryResolveAssignedValue(
+    const UIStyleValue& assignedValue,
+    UIStyleValueType expectedType,
+    const UITheme* theme,
+    UIStyleValue& outValue) {
+    if (!assignedValue.IsSet()) {
+        return false;
+    }
+
+    if (const UITokenReference* tokenReference = assignedValue.TryGetTokenReference()) {
+        if (theme == nullptr || !tokenReference->IsValid()) {
+            return false;
+        }
+
+        const UITokenResolveResult tokenResult = theme->ResolveToken(tokenReference->name, expectedType);
+        if (tokenResult.status != UITokenResolveStatus::Resolved) {
+            return false;
+        }
+
+        outValue = tokenResult.value;
+        return true;
+    }
+
+    if (expectedType != UIStyleValueType::None && assignedValue.GetType() != expectedType) {
+        return false;
+    }
+
+    outValue = assignedValue;
+    return true;
+}
+
+const UIStyleSet* GetStyleSetForLayer(UIStyleLayer layer, const UIStyleResolveContext& context) {
+    if (context.styleSheet == nullptr) {
+        return layer == UIStyleLayer::Local ? context.localStyle : nullptr;
+    }
+
+    switch (layer) {
+    case UIStyleLayer::Default:
+        return &context.styleSheet->DefaultStyle();
+    case UIStyleLayer::Type:
+        return context.styleSheet->FindTypeStyle(context.selector.typeName);
+    case UIStyleLayer::Named:
+        return context.styleSheet->FindNamedStyle(context.selector.styleName);
+    case UIStyleLayer::Local:
+        return context.localStyle;
+    default:
+        return nullptr;
+    }
+}
+
+} // namespace
+
+UIStylePropertyResolution ResolveStyleProperty(
+    UIStylePropertyId propertyId,
+    const UIStyleResolveContext& context) {
+    UIStylePropertyResolution resolution = {};
+    const UIStyleValueType expectedType = GetExpectedValueType(propertyId);
+    const UIStyleLayer lookupOrder[] = {
+        UIStyleLayer::Local,
+        UIStyleLayer::Named,
+        UIStyleLayer::Type,
+        UIStyleLayer::Default
+    };
+
+    for (const UIStyleLayer layer : lookupOrder) {
+        const UIStyleSet* styleSet = GetStyleSetForLayer(layer, context);
+        if (styleSet == nullptr) {
+            continue;
+        }
+
+        const UIStyleValue* assignedValue = styleSet->FindProperty(propertyId);
+        if (assignedValue == nullptr) {
+            continue;
+        }
+
+        UIStyleValue resolvedValue = {};
+        if (!TryResolveAssignedValue(*assignedValue, expectedType, context.theme, resolvedValue)) {
+            continue;
+        }
+
+        resolution.resolved = true;
+        resolution.layer = layer;
+        resolution.value = resolvedValue;
+        return resolution;
+    }
+
+    return resolution;
+}
+
+UIResolvedStyle ResolveStyle(const UIStyleResolveContext& context) {
+    UIResolvedStyle resolvedStyle = {};
+    const UIStylePropertyId knownProperties[] = {
+        UIStylePropertyId::BackgroundColor,
+        UIStylePropertyId::ForegroundColor,
+        UIStylePropertyId::BorderColor,
+        UIStylePropertyId::BorderWidth,
+        UIStylePropertyId::CornerRadius,
+        UIStylePropertyId::Padding,
+        UIStylePropertyId::Gap,
+        UIStylePropertyId::FontSize,
+        UIStylePropertyId::LineWidth
+    };
+
+    for (const UIStylePropertyId propertyId : knownProperties) {
+        const UIStylePropertyResolution propertyResolution = ResolveStyleProperty(propertyId, context);
+        if (propertyResolution.resolved) {
+            resolvedStyle.SetProperty(propertyId, propertyResolution);
+        }
+    }
+
+    return resolvedStyle;
+}
+
+const char* ToString(UIStyleLayer layer) {
+    switch (layer) {
+    case UIStyleLayer::Default:
+        return "Default";
+    case UIStyleLayer::Type:
+        return "Type";
+    case UIStyleLayer::Named:
+        return "Named";
+    case UIStyleLayer::Local:
+        return "Local";
+    default:
+        return "None";
+    }
+}
+
+} // namespace Style
+} // namespace UI
+} // namespace XCEngine
diff --git a/engine/src/UI/Style/StyleTypes.cpp b/engine/src/UI/Style/StyleTypes.cpp
new file mode 100644
index 00000000..bbf6da0d
--- /dev/null
+++ b/engine/src/UI/Style/StyleTypes.cpp
@@ -0,0 +1,245 @@
+#include <XCEngine/UI/Style/StyleTypes.h>
+
+#include <cmath>
+
+namespace XCEngine {
+namespace UI {
+namespace Style {
+
+namespace {
+
+bool NearlyEqual(float left, float right) {
+    return std::abs(left - right) <= Math::EPSILON;
+}
+
+bool ColorsEqual(const Math::Color& left, const Math::Color& right) {
+    return NearlyEqual(left.r, right.r) &&
+        NearlyEqual(left.g, right.g) &&
+        NearlyEqual(left.b, right.b) &&
+        NearlyEqual(left.a, right.a);
+}
+
+bool ThicknessEqual(const UIThickness& left, const UIThickness& right) {
+    return NearlyEqual(left.left, right.left) &&
+        NearlyEqual(left.top, right.top) &&
+        NearlyEqual(left.right, right.right) &&
+        NearlyEqual(left.bottom, right.bottom);
+}
+
+bool CornerRadiusEqual(const UICornerRadius& left, const UICornerRadius& right) {
+    return NearlyEqual(left.topLeft, right.topLeft) &&
+        NearlyEqual(left.topRight, right.topRight) &&
+        NearlyEqual(left.bottomRight, right.bottomRight) &&
+        NearlyEqual(left.bottomLeft, right.bottomLeft);
+}
+
+bool SizeEqual(const UISize& left, const UISize& right) {
+    return NearlyEqual(left.width, right.width) &&
+        NearlyEqual(left.height, right.height);
+}
+
+bool PointEqual(const UIPoint& left, const UIPoint& right) {
+    return NearlyEqual(left.x, right.x) &&
+        NearlyEqual(left.y, right.y);
+}
+
+} // namespace
+
+UIThickness UIThickness::Uniform(float value) {
+    return { value, value, value, value };
+}
+
+bool UIThickness::IsUniform() const {
+    return NearlyEqual(left, top) &&
+        NearlyEqual(left, right) &&
+        NearlyEqual(left, bottom);
+}
+
+UICornerRadius UICornerRadius::Uniform(float value) {
+    return { value, value, value, value };
+}
+
+bool UICornerRadius::IsUniform() const {
+    return NearlyEqual(topLeft, topRight) &&
+        NearlyEqual(topLeft, bottomRight) &&
+        NearlyEqual(topLeft, bottomLeft);
+}
+
+bool UITokenReference::IsValid() const {
+    return !name.empty();
+}
+
+UIStyleValue::UIStyleValue(float value)
+    : m_storage(value) {
+}
+
+UIStyleValue::UIStyleValue(const Math::Color& value)
+    : m_storage(value) {
+}
+
+UIStyleValue::UIStyleValue(const UIThickness& value)
+    : m_storage(value) {
+}
+
+UIStyleValue::UIStyleValue(const UICornerRadius& value)
+    : m_storage(value) {
+}
+
+UIStyleValue::UIStyleValue(const UISize& value)
+    : m_storage(value) {
+}
+
+UIStyleValue::UIStyleValue(const UIPoint& value)
+    : m_storage(value) {
+}
+
+UIStyleValue::UIStyleValue(const UITokenReference& tokenReference)
+    : m_storage(tokenReference) {
+}
+
+UIStyleValue UIStyleValue::Token(const std::string& tokenName) {
+    return UIStyleValue(UITokenReference{ tokenName });
+}
+
+bool UIStyleValue::IsSet() const {
+    return !std::holds_alternative<std::monostate>(m_storage);
+}
+
+bool UIStyleValue::IsTokenReference() const {
+    return std::holds_alternative<UITokenReference>(m_storage);
+}
+
+UIStyleValueType UIStyleValue::GetType() const {
+    if (std::holds_alternative<float>(m_storage)) {
+        return UIStyleValueType::Float;
+    }
+    if (std::holds_alternative<Math::Color>(m_storage)) {
+        return UIStyleValueType::Color;
+    }
+    if (std::holds_alternative<UIThickness>(m_storage)) {
+        return UIStyleValueType::Thickness;
+    }
+    if (std::holds_alternative<UICornerRadius>(m_storage)) {
+        return UIStyleValueType::CornerRadius;
+    }
+    if (std::holds_alternative<UISize>(m_storage)) {
+        return UIStyleValueType::Size;
+    }
+    if (std::holds_alternative<UIPoint>(m_storage)) {
+        return UIStyleValueType::Point;
+    }
+    if (std::holds_alternative<UITokenReference>(m_storage)) {
+        return UIStyleValueType::TokenReference;
+    }
+    return UIStyleValueType::None;
+}
+
+const float* UIStyleValue::TryGetFloat() const {
+    return std::get_if<float>(&m_storage);
+}
+
+const Math::Color* UIStyleValue::TryGetColor() const {
+    return std::get_if<Math::Color>(&m_storage);
+}
+
+const UIThickness* UIStyleValue::TryGetThickness() const {
+    return std::get_if<UIThickness>(&m_storage);
+}
+
+const UICornerRadius* UIStyleValue::TryGetCornerRadius() const {
+    return std::get_if<UICornerRadius>(&m_storage);
+}
+
+const UISize* UIStyleValue::TryGetSize() const {
+    return std::get_if<UISize>(&m_storage);
+}
+
+const UIPoint* UIStyleValue::TryGetPoint() const {
+    return std::get_if<UIPoint>(&m_storage);
+}
+
+const UITokenReference* UIStyleValue::TryGetTokenReference() const {
+    return std::get_if<UITokenReference>(&m_storage);
+}
+
+bool UIStyleValue::operator==(const UIStyleValue& other) const {
+    if (GetType() != other.GetType()) {
+        return false;
+    }
+
+    if (const float* left = TryGetFloat()) {
+        return NearlyEqual(*left, *other.TryGetFloat());
+    }
+    if (const Math::Color* left = TryGetColor()) {
+        return ColorsEqual(*left, *other.TryGetColor());
+    }
+    if (const UIThickness* left = TryGetThickness()) {
+        return ThicknessEqual(*left, *other.TryGetThickness());
+    }
+    if (const UICornerRadius* left = TryGetCornerRadius()) {
+        return CornerRadiusEqual(*left, *other.TryGetCornerRadius());
+    }
+    if (const UISize* left = TryGetSize()) {
+        return SizeEqual(*left, *other.TryGetSize());
+    }
+    if (const UIPoint* left = TryGetPoint()) {
+        return PointEqual(*left, *other.TryGetPoint());
+    }
+    if (const UITokenReference* left = TryGetTokenReference()) {
+        return left->name == other.TryGetTokenReference()->name;
+    }
+    return true;
+}
+
+bool UIStyleValue::operator!=(const UIStyleValue& other) const {
+    return !(*this == other);
+}
+
+UIStyleValueType GetExpectedValueType(UIStylePropertyId propertyId) {
+    switch (propertyId) {
+    case UIStylePropertyId::BackgroundColor:
+    case UIStylePropertyId::ForegroundColor:
+    case UIStylePropertyId::BorderColor:
+        return UIStyleValueType::Color;
+    case UIStylePropertyId::BorderWidth:
+    case UIStylePropertyId::Gap:
+    case UIStylePropertyId::FontSize:
+    case UIStylePropertyId::LineWidth:
+        return UIStyleValueType::Float;
+    case UIStylePropertyId::CornerRadius:
+        return UIStyleValueType::CornerRadius;
+    case UIStylePropertyId::Padding:
+        return UIStyleValueType::Thickness;
+    default:
+        return UIStyleValueType::None;
+    }
+}
+
+const char* ToString(UIStylePropertyId propertyId) {
+    switch (propertyId) {
+    case UIStylePropertyId::BackgroundColor:
+        return "BackgroundColor";
+    case UIStylePropertyId::ForegroundColor:
+        return "ForegroundColor";
+    case UIStylePropertyId::BorderColor:
+        return "BorderColor";
+    case UIStylePropertyId::BorderWidth:
+        return "BorderWidth";
+    case UIStylePropertyId::CornerRadius:
+        return "CornerRadius";
+    case UIStylePropertyId::Padding:
+        return "Padding";
+    case UIStylePropertyId::Gap:
+        return "Gap";
+    case UIStylePropertyId::FontSize:
+        return "FontSize";
+    case UIStylePropertyId::LineWidth:
+        return "LineWidth";
+    default:
+        return "Unknown";
+    }
+}
+
+} // namespace Style
+} // namespace UI
+} // namespace XCEngine
diff --git a/engine/src/UI/Style/Theme.cpp b/engine/src/UI/Style/Theme.cpp
new file mode 100644
index 00000000..a342e29d
--- /dev/null
+++ b/engine/src/UI/Style/Theme.cpp
@@ -0,0 +1,145 @@
+#include <XCEngine/UI/Style/Theme.h>
+
+#include <algorithm>
+
+namespace XCEngine {
+namespace UI {
+namespace Style {
+
+void UIThemeDefinition::SetToken(const std::string& tokenName, const UIStyleValue& value) {
+    tokens[tokenName] = value;
+}
+
+const UIStyleValue* UIThemeDefinition::FindToken(const std::string& tokenName) const {
+    const auto it = tokens.find(tokenName);
+    return it != tokens.end() ? &it->second : nullptr;
+}
+
+UITheme::UITheme(const UIThemeDefinition& definition)
+    : m_name(definition.name)
+    , m_tokens(definition.tokens) {
+}
+
+void UITheme::SetName(const std::string& name) {
+    m_name = name;
+}
+
+const std::string& UITheme::GetName() const {
+    return m_name;
+}
+
+void UITheme::SetParent(const UITheme* parent) {
+    m_parent = parent;
+}
+
+const UITheme* UITheme::GetParent() const {
+    return m_parent;
+}
+
+void UITheme::SetToken(const std::string& tokenName, const UIStyleValue& value) {
+    m_tokens[tokenName] = value;
+}
+
+bool UITheme::RemoveToken(const std::string& tokenName) {
+    return m_tokens.erase(tokenName) > 0u;
+}
+
+const UIStyleValue* UITheme::FindToken(const std::string& tokenName) const {
+    const auto it = m_tokens.find(tokenName);
+    if (it != m_tokens.end()) {
+        return &it->second;
+    }
+
+    return m_parent != nullptr ? m_parent->FindToken(tokenName) : nullptr;
+}
+
+const std::map<std::string, UIStyleValue>& UITheme::GetTokens() const {
+    return m_tokens;
+}
+
+UITokenResolveResult UITheme::ResolveToken(
+    const std::string& tokenName,
+    UIStyleValueType expectedType) const {
+    std::vector<std::string> visiting = {};
+    return ResolveTokenRecursive(tokenName, expectedType, visiting);
+}
+
+UITokenResolveResult UITheme::ResolveTokenRecursive(
+    const std::string& tokenName,
+    UIStyleValueType expectedType,
+    std::vector<std::string>& visiting) const {
+    UITokenResolveResult result = {};
+    if (tokenName.empty()) {
+        result.status = UITokenResolveStatus::MissingToken;
+        return result;
+    }
+
+    if (std::find(visiting.begin(), visiting.end(), tokenName) != visiting.end()) {
+        result.status = UITokenResolveStatus::CycleDetected;
+        return result;
+    }
+
+    const UIStyleValue* value = FindToken(tokenName);
+    if (value == nullptr || !value->IsSet()) {
+        result.status = UITokenResolveStatus::MissingToken;
+        return result;
+    }
+
+    if (const UITokenReference* tokenReference = value->TryGetTokenReference()) {
+        visiting.push_back(tokenName);
+        result = ResolveTokenRecursive(tokenReference->name, expectedType, visiting);
+        visiting.pop_back();
+        return result;
+    }
+
+    if (expectedType != UIStyleValueType::None && value->GetType() != expectedType) {
+        result.status = UITokenResolveStatus::TypeMismatch;
+        return result;
+    }
+
+    result.status = UITokenResolveStatus::Resolved;
+    result.value = *value;
+    return result;
+}
+
+UITheme BuildTheme(const UIThemeDefinition& definition, const UITheme* parent) {
+    UITheme theme(definition);
+    theme.SetParent(parent);
+    return theme;
+}
+
+UITheme BuildBuiltinTheme(UIBuiltinThemeKind themeKind) {
+    UIThemeDefinition definition = {};
+
+    if (themeKind == UIBuiltinThemeKind::NeutralLight) {
+        definition.name = "NeutralLight";
+        definition.SetToken("color.surface", UIStyleValue(Math::Color(0.93f, 0.94f, 0.96f, 1.0f)));
+        definition.SetToken("color.surface.elevated", UIStyleValue(Math::Color(1.0f, 1.0f, 1.0f, 1.0f)));
+        definition.SetToken("color.text.primary", UIStyleValue(Math::Color(0.10f, 0.11f, 0.14f, 1.0f)));
+        definition.SetToken("color.accent", UIStyleValue(Math::Color(0.14f, 0.43f, 0.88f, 1.0f)));
+        definition.SetToken("space.compact", UIStyleValue(6.0f));
+        definition.SetToken("space.regular", UIStyleValue(10.0f));
+        definition.SetToken("padding.control", UIStyleValue(UIThickness::Uniform(10.0f)));
+        definition.SetToken("radius.control", UIStyleValue(UICornerRadius::Uniform(6.0f)));
+        definition.SetToken("font.body", UIStyleValue(14.0f));
+        definition.SetToken("line.default", UIStyleValue(1.0f));
+        return BuildTheme(definition);
+    }
+
+    definition.name = "NeutralDark";
+    definition.SetToken("color.surface", UIStyleValue(Math::Color(0.13f, 0.14f, 0.16f, 1.0f)));
+    definition.SetToken("color.surface.elevated", UIStyleValue(Math::Color(0.18f, 0.19f, 0.22f, 1.0f)));
+    definition.SetToken("color.text.primary", UIStyleValue(Math::Color(0.92f, 0.93f, 0.95f, 1.0f)));
+    definition.SetToken("color.accent", UIStyleValue(Math::Color(0.98f, 0.53f, 0.17f, 1.0f)));
+    definition.SetToken("space.compact", UIStyleValue(6.0f));
+    definition.SetToken("space.regular", UIStyleValue(10.0f));
+    definition.SetToken("padding.control", UIStyleValue(UIThickness::Uniform(10.0f)));
+    definition.SetToken("radius.control", UIStyleValue(UICornerRadius::Uniform(6.0f)));
+    definition.SetToken("font.body", UIStyleValue(14.0f));
+    definition.SetToken("line.default", UIStyleValue(1.0f));
+    return BuildTheme(definition);
+}
+
+} // namespace Style
+} // namespace UI
+} // namespace XCEngine
diff --git a/tests/core/CMakeLists.txt b/tests/core/CMakeLists.txt
index adc2a49b..632a452d 100644
--- a/tests/core/CMakeLists.txt
+++ b/tests/core/CMakeLists.txt
@@ -32,6 +32,7 @@ add_subdirectory(Math)
 # Core/UI Tests
 # ============================================================
 add_subdirectory(UI)
+add_subdirectory(UIStyle)
 
 # Exclude all static runtime libraries to avoid conflicts
 if(MSVC)
diff --git a/tests/core/UIStyle/CMakeLists.txt b/tests/core/UIStyle/CMakeLists.txt
new file mode 100644
index 00000000..62db5ff8
--- /dev/null
+++ b/tests/core/UIStyle/CMakeLists.txt
@@ -0,0 +1,30 @@
+# ============================================================
+# UI Style Tests
+# ============================================================
+
+set(UI_STYLE_TEST_SOURCES
+    test_style_system.cpp
+)
+
+add_executable(core_ui_style_tests ${UI_STYLE_TEST_SOURCES})
+
+if(MSVC)
+    set_target_properties(core_ui_style_tests PROPERTIES
+        LINK_FLAGS "/NODEFAULTLIB:libcpmt.lib /NODEFAULTLIB:libcmt.lib"
+    )
+endif()
+
+target_link_libraries(core_ui_style_tests
+    PRIVATE
+    XCEngine
+    GTest::gtest
+    GTest::gtest_main
+)
+
+target_include_directories(core_ui_style_tests PRIVATE
+    ${CMAKE_SOURCE_DIR}/engine/include
+    ${CMAKE_SOURCE_DIR}/tests/Fixtures
+)
+
+include(GoogleTest)
+gtest_discover_tests(core_ui_style_tests)
diff --git a/tests/core/UIStyle/test_style_system.cpp b/tests/core/UIStyle/test_style_system.cpp
new file mode 100644
index 00000000..9f99b3e7
--- /dev/null
+++ b/tests/core/UIStyle/test_style_system.cpp
@@ -0,0 +1,166 @@
+#include <gtest/gtest.h>
+
+#include <XCEngine/UI/Style/StyleResolver.h>
+
+namespace {
+
+using XCEngine::Math::Color;
+using XCEngine::UI::Style::BuildBuiltinTheme;
+using XCEngine::UI::Style::BuildTheme;
+using XCEngine::UI::Style::UICornerRadius;
+using XCEngine::UI::Style::UIBuiltinThemeKind;
+using XCEngine::UI::Style::UIResolvedStyle;
+using XCEngine::UI::Style::UIStyleLayer;
+using XCEngine::UI::Style::UIStylePropertyId;
+using XCEngine::UI::Style::UIStyleResolveContext;
+using XCEngine::UI::Style::UIStyleSet;
+using XCEngine::UI::Style::UIStyleSheet;
+using XCEngine::UI::Style::UIStyleValue;
+using XCEngine::UI::Style::UIStyleValueType;
+using XCEngine::UI::Style::UITheme;
+using XCEngine::UI::Style::UIThemeDefinition;
+using XCEngine::UI::Style::UITokenResolveStatus;
+
+void ExpectColorEq(const Color& actual, const Color& expected) {
+    EXPECT_FLOAT_EQ(actual.r, expected.r);
+    EXPECT_FLOAT_EQ(actual.g, expected.g);
+    EXPECT_FLOAT_EQ(actual.b, expected.b);
+    EXPECT_FLOAT_EQ(actual.a, expected.a);
+}
+
+TEST(UIStyleSystem, ThemeDefinitionBuildsThemeAndResolvesAliases) {
+    UIThemeDefinition definition = {};
+    definition.name = "CustomTheme";
+    definition.SetToken("space.base", UIStyleValue(8.0f));
+    definition.SetToken("gap.control", UIStyleValue::Token("space.base"));
+    definition.SetToken("radius.card", UIStyleValue(UICornerRadius::Uniform(12.0f)));
+
+    const UITheme theme = BuildTheme(definition);
+
+    EXPECT_EQ(theme.GetName(), "CustomTheme");
+
+    const auto gapToken = theme.ResolveToken("gap.control", UIStyleValueType::Float);
+    ASSERT_EQ(gapToken.status, UITokenResolveStatus::Resolved);
+    ASSERT_NE(gapToken.value.TryGetFloat(), nullptr);
+    EXPECT_FLOAT_EQ(*gapToken.value.TryGetFloat(), 8.0f);
+
+    const auto radiusToken = theme.ResolveToken("radius.card", UIStyleValueType::CornerRadius);
+    ASSERT_EQ(radiusToken.status, UITokenResolveStatus::Resolved);
+    ASSERT_NE(radiusToken.value.TryGetCornerRadius(), nullptr);
+    EXPECT_TRUE(radiusToken.value.TryGetCornerRadius()->IsUniform());
+    EXPECT_FLOAT_EQ(radiusToken.value.TryGetCornerRadius()->topLeft, 12.0f);
+}
+
+TEST(UIStyleSystem, ThemeResolvesParentTokensAndBuiltinVariantsDiffer) {
+    UIThemeDefinition baseDefinition = {};
+    baseDefinition.name = "Base";
+    baseDefinition.SetToken("color.surface", UIStyleValue(Color(0.20f, 0.21f, 0.22f, 1.0f)));
+    const UITheme baseTheme = BuildTheme(baseDefinition);
+
+    UIThemeDefinition childDefinition = {};
+    childDefinition.name = "Child";
+    childDefinition.SetToken("font.body", UIStyleValue(15.0f));
+    const UITheme childTheme = BuildTheme(childDefinition, &baseTheme);
+
+    const auto parentToken = childTheme.ResolveToken("color.surface", UIStyleValueType::Color);
+    ASSERT_EQ(parentToken.status, UITokenResolveStatus::Resolved);
+    ASSERT_NE(parentToken.value.TryGetColor(), nullptr);
+    ExpectColorEq(*parentToken.value.TryGetColor(), Color(0.20f, 0.21f, 0.22f, 1.0f));
+
+    const UITheme darkTheme = BuildBuiltinTheme(UIBuiltinThemeKind::NeutralDark);
+    const UITheme lightTheme = BuildBuiltinTheme(UIBuiltinThemeKind::NeutralLight);
+    const auto darkSurface = darkTheme.ResolveToken("color.surface", UIStyleValueType::Color);
+    const auto lightSurface = lightTheme.ResolveToken("color.surface", UIStyleValueType::Color);
+    ASSERT_EQ(darkSurface.status, UITokenResolveStatus::Resolved);
+    ASSERT_EQ(lightSurface.status, UITokenResolveStatus::Resolved);
+    ASSERT_NE(darkSurface.value.TryGetColor(), nullptr);
+    ASSERT_NE(lightSurface.value.TryGetColor(), nullptr);
+    EXPECT_NE(darkSurface.value.TryGetColor()->r, lightSurface.value.TryGetColor()->r);
+}
+
+TEST(UIStyleSystem, ThemeReportsMissingCyclesAndTypeMismatches) {
+    UIThemeDefinition definition = {};
+    definition.SetToken("cycle.a", UIStyleValue::Token("cycle.b"));
+    definition.SetToken("cycle.b", UIStyleValue::Token("cycle.a"));
+    definition.SetToken("color.surface", UIStyleValue(Color(0.1f, 0.2f, 0.3f, 1.0f)));
+    const UITheme theme = BuildTheme(definition);
+
+    EXPECT_EQ(
+        theme.ResolveToken("missing.token", UIStyleValueType::Float).status,
+        UITokenResolveStatus::MissingToken);
+    EXPECT_EQ(
+        theme.ResolveToken("cycle.a", UIStyleValueType::Float).status,
+        UITokenResolveStatus::CycleDetected);
+    EXPECT_EQ(
+        theme.ResolveToken("color.surface", UIStyleValueType::Float).status,
+        UITokenResolveStatus::TypeMismatch);
+}
+
+TEST(UIStyleSystem, StyleResolutionPrefersLocalThenNamedThenTypeThenDefault) {
+    UIStyleSheet styleSheet = {};
+    styleSheet.DefaultStyle().SetProperty(UIStylePropertyId::FontSize, UIStyleValue(12.0f));
+    styleSheet.GetOrCreateTypeStyle("Button").SetProperty(UIStylePropertyId::FontSize, UIStyleValue(14.0f));
+    styleSheet.GetOrCreateNamedStyle("Primary").SetProperty(UIStylePropertyId::FontSize, UIStyleValue(16.0f));
+
+    UIStyleSet localStyle = {};
+    localStyle.SetProperty(UIStylePropertyId::FontSize, UIStyleValue(18.0f));
+
+    UIStyleResolveContext context = {};
+    context.styleSheet = &styleSheet;
+    context.selector.typeName = "Button";
+    context.selector.styleName = "Primary";
+    context.localStyle = &localStyle;
+
+    auto resolution = XCEngine::UI::Style::ResolveStyleProperty(UIStylePropertyId::FontSize, context);
+    ASSERT_TRUE(resolution.resolved);
+    EXPECT_EQ(resolution.layer, UIStyleLayer::Local);
+    ASSERT_NE(resolution.value.TryGetFloat(), nullptr);
+    EXPECT_FLOAT_EQ(*resolution.value.TryGetFloat(), 18.0f);
+
+    localStyle.RemoveProperty(UIStylePropertyId::FontSize);
+    resolution = XCEngine::UI::Style::ResolveStyleProperty(UIStylePropertyId::FontSize, context);
+    ASSERT_TRUE(resolution.resolved);
+    EXPECT_EQ(resolution.layer, UIStyleLayer::Named);
+    EXPECT_FLOAT_EQ(*resolution.value.TryGetFloat(), 16.0f);
+
+    styleSheet.GetOrCreateNamedStyle("Primary").RemoveProperty(UIStylePropertyId::FontSize);
+    resolution = XCEngine::UI::Style::ResolveStyleProperty(UIStylePropertyId::FontSize, context);
+    ASSERT_TRUE(resolution.resolved);
+    EXPECT_EQ(resolution.layer, UIStyleLayer::Type);
+    EXPECT_FLOAT_EQ(*resolution.value.TryGetFloat(), 14.0f);
+}
+
+TEST(UIStyleSystem, StyleResolutionFallsBackWhenHigherPriorityTokenCannotResolve) {
+    UIThemeDefinition themeDefinition = {};
+    themeDefinition.SetToken("color.accent", UIStyleValue(Color(0.90f, 0.20f, 0.10f, 1.0f)));
+    const UITheme theme = BuildTheme(themeDefinition);
+
+    UIStyleSheet styleSheet = {};
+    styleSheet.DefaultStyle().SetProperty(UIStylePropertyId::BorderWidth, UIStyleValue(1.0f));
+    styleSheet.GetOrCreateTypeStyle("Button")
+        .SetProperty(UIStylePropertyId::BackgroundColor, UIStyleValue::Token("color.accent"));
+    styleSheet.GetOrCreateNamedStyle("Danger")
+        .SetProperty(UIStylePropertyId::BackgroundColor, UIStyleValue::Token("missing.token"));
+
+    UIStyleResolveContext context = {};
+    context.theme = &theme;
+    context.styleSheet = &styleSheet;
+    context.selector.typeName = "Button";
+    context.selector.styleName = "Danger";
+
+    const auto backgroundResolution =
+        XCEngine::UI::Style::ResolveStyleProperty(UIStylePropertyId::BackgroundColor, context);
+    ASSERT_TRUE(backgroundResolution.resolved);
+    EXPECT_EQ(backgroundResolution.layer, UIStyleLayer::Type);
+    ASSERT_NE(backgroundResolution.value.TryGetColor(), nullptr);
+    ExpectColorEq(*backgroundResolution.value.TryGetColor(), Color(0.90f, 0.20f, 0.10f, 1.0f));
+
+    const UIResolvedStyle resolvedStyle = XCEngine::UI::Style::ResolveStyle(context);
+    const auto* borderWidthResolution = resolvedStyle.FindProperty(UIStylePropertyId::BorderWidth);
+    ASSERT_NE(borderWidthResolution, nullptr);
+    EXPECT_EQ(borderWidthResolution->layer, UIStyleLayer::Default);
+    ASSERT_NE(borderWidthResolution->value.TryGetFloat(), nullptr);
+    EXPECT_FLOAT_EQ(*borderWidthResolution->value.TryGetFloat(), 1.0f);
+}
+
+} // namespace