Add functional Theme selection, with Provider for state management

frontend_splatournament_manager/lib/widgets/theme_selector_widget.dart

@@ -0,0 +1,52 @@
+import 'package:flutter/material.dart';
+import 'package:provider/provider.dart';
+
+import '../state_provider.dart';
+
+class ThemeSelectorWidget extends StatelessWidget {
+  ThemeSelectorWidget({super.key});
+
+  final List<DropdownMenuItem> dropdownElements = [
+    DropdownMenuItem(
+      value: ThemeMode.light,
+      child: Text("Light"),
+    ),
+    DropdownMenuItem(
+      value: ThemeMode.dark,
+      child: Text("Dark"),
+    ),
+    DropdownMenuItem(
+      value: ThemeMode.system,
+      child: Text("System"),
+    ),
+  ];
+
+  @override
+  Widget build(BuildContext context) {
+    final stateProvider = Provider.of<StateProvider>(context);
+    return Container(
+      decoration: BoxDecoration(color: Theme.of(context).hoverColor, borderRadius: BorderRadius.circular(8)),
+      padding: const EdgeInsets.all(8.0),
+      margin: const EdgeInsets.all(8.0),
+      child: Row(
+        mainAxisAlignment: MainAxisAlignment.spaceBetween,
+        children: [
+          Text("Theme"),
+          SizedBox(
+            width: 250,
+            child: DropdownButtonFormField(
+              icon: Icon(Icons.color_lens),
+              items: dropdownElements,
+              initialValue: stateProvider.theme,
+              onChanged: (value) {
+                if (value == null) return;
+                stateProvider.setTheme(value);
+              },
+            ),
+          ),
+        ],
+      ),
+    );
+  }
+
+}