feat: Windows system title bar theme follows the theme

lib/app_widget.dart

@@ -164,6 +164,21 @@ class _AppWidgetState extends State<AppWidget>
     }
   }
 
+  @override
+  Future<void> didChangePlatformBrightness() async {
+    super.didChangePlatformBrightness();
+    final ThemeProvider themeProvider = Provider.of<ThemeProvider>(context, listen: false);
+    KazumiLogger().i("Platform brightness changed, themeMode: ${themeProvider.themeMode}");
+
+    // Only update title bar theme when following system
+    // If user has forced a specific theme, keep title bar consistent with app content
+    if (themeProvider.themeMode == ThemeMode.system && Platform.isWindows) {
+      final brightness = WidgetsBinding.instance.platformDispatcher.platformBrightness;
+      KazumiLogger().i("Updating title bar brightness: $brightness");
+      await windowManager.setBrightness(brightness);
+    }
+  }
+
   Future<void> _handleTray() async {
     if (Platform.isWindows) {
       await trayManager.setIcon('assets/images/logo/logo_lanczos.ico');
@@ -215,6 +230,12 @@ class _AppWidgetState extends State<AppWidget>
     if (defaultThemeMode == 'system') {
       themeProvider.setThemeMode(ThemeMode.system, notify: false);
     }
+
+    // Set Windows title bar theme based on app theme
+    if (Platform.isWindows) {
+      windowManager.setBrightness(themeProvider.isEffectiveDark() ? Brightness.dark : Brightness.light);
+    }
+
     themeProvider.setFontFamily(useSystemFont, notify: false);
     var defaultDarkTheme = ThemeData(
         useMaterial3: true,

lib/bean/settings/theme_provider.dart

@@ -1,4 +1,5 @@
 import 'package:flutter/material.dart';
+import 'package:flutter/scheduler.dart';
 import 'package:kazumi/utils/constants.dart';
 
 class ThemeProvider extends ChangeNotifier {
@@ -8,6 +9,15 @@ class ThemeProvider extends ChangeNotifier {
   late ThemeData dark;
   String? currentFontFamily = customAppFontFamily;
 
+  /// Returns true if the effective theme is dark mode.
+  /// Automatically gets platform brightness when themeMode is ThemeMode.system.
+  bool isEffectiveDark() {
+    if (themeMode == ThemeMode.dark) return true;
+    if (themeMode == ThemeMode.light) return false;
+    final platformBrightness = SchedulerBinding.instance.platformDispatcher.platformBrightness;
+    return platformBrightness == Brightness.dark;
+  }
+
   void setTheme(ThemeData light, ThemeData dark, {bool notify = true}) {
     this.light = light;
     this.dark = dark;

lib/pages/settings/theme_settings_page.dart

@@ -13,6 +13,7 @@ import 'package:kazumi/bean/settings/color_type.dart';
 import 'package:kazumi/utils/utils.dart';
 import 'package:card_settings_ui/card_settings_ui.dart';
 import 'package:provider/provider.dart';
+import 'package:window_manager/window_manager.dart';
 
 class ThemeSettingsPage extends StatefulWidget {
   const ThemeSettingsPage({super.key});
@@ -122,6 +123,11 @@ class _ThemeSettingsPageState extends State<ThemeSettingsPage> {
     setState(() {
       defaultThemeMode = theme;
     });
+
+    // Update Windows title bar theme
+    if (Platform.isWindows) {
+      await windowManager.setBrightness(themeProvider.isEffectiveDark() ? Brightness.dark : Brightness.light);
+    }
   }
 
   void updateOledEnhance() {

windows/runner/win32_window.cpp

@@ -221,9 +221,10 @@ Win32Window::MessageHandler(HWND hwnd,
       }
       return 0;
 
-    case WM_DWMCOLORIZATIONCOLORCHANGED:
-      UpdateTheme(hwnd);
-      return 0;
+    // Theme state is now maintained by Flutter side (app_widget.dart).
+    // case WM_DWMCOLORIZATIONCOLORCHANGED:
+    //   UpdateTheme(hwnd);
+    //   return 0;
   }
 
   return DefWindowProc(window_handle_, message, wparam, lparam);