update dark mode handling

frontend/lib/main.dart

@@ -1,4 +1,5 @@
 import 'package:flutter/material.dart';
+import 'package:frontend/provider/theme_provider.dart';
 import 'package:go_router/go_router.dart';
 import 'package:provider/provider.dart';
 
@@ -17,10 +18,17 @@ import 'pages/leaderboard.dart';
 
 void main() {
   runApp(
-    ChangeNotifierProvider(
-      create: (_) => QuoteProvider(),
-      child: HlaskovnikFrontendApp(),
-    ),
+    MultiProvider(
+      providers: [
+        ChangeNotifierProvider(
+          create: (_) => QuoteProvider()
+        ),
+        ChangeNotifierProvider(
+          create: (_) => ThemeProvider()
+        )
+      ],
+      child: HlaskovnikFrontendApp()
+    )
   );
 }
 
@@ -46,6 +54,7 @@ class HlaskovnikFrontendApp extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
+    final themeProvider = context.watch<ThemeProvider>();
     return MaterialApp.router(
       title: Strings.appName,
       theme: ThemeData(
@@ -62,7 +71,7 @@ class HlaskovnikFrontendApp extends StatelessWidget {
         ),
         useMaterial3: true,
       ),
-      themeMode: ThemeMode.system,
+      themeMode: themeProvider.isDark ? ThemeMode.dark : ThemeMode.system,
       routerConfig: _router,
     );
   }