Add functional Theme selection, with Provider for state management

frontend_splatournament_manager/lib/state_provider.dart

@@ -0,0 +1,12 @@
+import 'package:flutter/material.dart';
+
+class StateProvider extends ChangeNotifier {
+  ThemeMode _themeMode = ThemeMode.system;
+
+  ThemeMode get theme => _themeMode;
+
+  void setTheme(ThemeMode mode) {
+    _themeMode = mode;
+    notifyListeners();
+  }
+}