How to Switch Between Themes in Flutter Using RadioButtons?

import 'package:flutter/material.dart';

void main() => runApp(const MyApp());

class MyApp extends StatefulWidget {
    const MyApp({Key? key}) : super(key: key);

    @override
    _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
    int _themeIndex = 0;
    final List<ThemeData> _themes = [
        ThemeData(
            primarySwatch: Colors.blue,
            scaffoldBackgroundColor: Colors.blue.shade100
        ),
        ThemeData(
            primarySwatch: Colors.red,
            scaffoldBackgroundColor: Colors.red.shade100
        ),
        ThemeData(
            primarySwatch: Colors.green,
            scaffoldBackgroundColor: Colors.green.shade100
        ),
        ThemeData(
            primarySwatch: Colors.purple,
            scaffoldBackgroundColor: Colors.purple.shade100
        ),
      ];

      @override
      Widget build(BuildContext context) {
        return MaterialApp(
              title: 'GFG - Flutter Themes',
              theme: _themes[_themeIndex],
              home: Scaffold(
                appBar: AppBar(
                      title: const Text('GFG - Flutter Themes'),
                ),
                body: Center(
                      child: Column(
                           mainAxisAlignment: MainAxisAlignment.center,
                        children: [
                              RadioListTile(
                                title: const Text('Blue Theme'),
                                value: 0,
                                groupValue: _themeIndex,
                                onChanged: _handleThemeChange,
                              ),
                              RadioListTile(
                                title: const Text('Red Theme'),
                                value: 1,
                                groupValue: _themeIndex,
                                onChanged: _handleThemeChange,
                              ),
                              RadioListTile(
                                title: const Text('Green Theme'),
                                value: 2,
                                groupValue: _themeIndex,
                                onChanged: _handleThemeChange,
                              ),
                              RadioListTile(
                                title: const Text('Purple Theme'),
                                value: 3,
                                groupValue: _themeIndex,
                                onChanged: _handleThemeChange,
                              ),
                        ],
                      ),
                ),
              ),
        );
      }

      void _handleThemeChange(int? value) {
        setState(() {
              _themeIndex = value!;
        });
      }
}