add dark mode

cupertino_gallery/lib/main.dart

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 import 'package:flutter/cupertino.dart';
+import 'package:flutter/material.dart';
 
 import 'gallery_home.dart';
 
@@ -10,11 +11,33 @@ void main() {
   runApp(const CupertinoGalleryApp());
 }
 
-class CupertinoGalleryApp extends StatelessWidget {
+class CupertinoGalleryApp extends StatefulWidget {
   const CupertinoGalleryApp({super.key});
 
+  @override
+  State<CupertinoGalleryApp> createState() => _CupertinoGalleryAppState();
+}
+
+class _CupertinoGalleryAppState extends State<CupertinoGalleryApp> {
+  ThemeMode _themeMode = ThemeMode.system;
+
+  void _handleThemeChange(bool isDarkMode) {
+    setState(() {
+      _themeMode = isDarkMode ? ThemeMode.dark : ThemeMode.light;
+    });
+  }
+
   @override
   Widget build(BuildContext context) {
-    return const CupertinoApp(title: 'Cupertino Gallery', home: GalleryHome());
+    return CupertinoApp(
+      title: 'Cupertino Gallery',
+      theme: CupertinoThemeData(
+        brightness: _themeMode == ThemeMode.dark ? Brightness.dark : Brightness.light,
+      ),
+      home: GalleryHome(
+        onThemeChange: _handleThemeChange,
+        isDarkMode: _themeMode == ThemeMode.dark,
+      ),
+    );
   }
 }