Add functional Theme selection, with Provider for state management

frontend_splatournament_manager/test/widget_test.dart

@@ -13,7 +13,7 @@ import 'package:frontend_splatournament_manager/main.dart';
 void main() {
   testWidgets('Counter increments smoke test', (WidgetTester tester) async {
     // Build our app and trigger a frame.
-    await tester.pumpWidget(const MyApp());
+    await tester.pumpWidget(const SplatournamentApp());
 
     // Verify that our counter starts at 0.
     expect(find.text('0'), findsOneWidget);