main, check for and use system theme properly.

lib/main.dart

@@ -30,12 +30,22 @@ class SunsetAlarmApp extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
+    const seedColor = Colors.deepOrange;
     return MaterialApp(
       title: 'Sunset Alarm',
       theme: ThemeData(
-        colorScheme: ColorScheme.fromSeed(seedColor: Colors.deepOrange),
-        useMaterial3: true,
+        colorScheme: ColorScheme.fromSeed(
+          seedColor: seedColor,
+          brightness: Brightness.light,
+        ),
       ),
+      darkTheme: ThemeData(
+        colorScheme: ColorScheme.fromSeed(
+          seedColor: seedColor,
+          brightness: Brightness.dark,
+        ),
+      ),
+      themeMode: ThemeMode.system,
       home: const HomeScreen(),
     );
   }