Using Toggle theme from System to light or System to dark with Consumer

theme_toggle.dart

import 'package:flutter/material.dart';
import 'package:provider/provider.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return ChangeNotifierProvider<Settings>(
      create: (context) => Settings(),
      child: Consumer<Settings>(builder: (context, appSettings, _) {
        return AnimatedBuilder(
            animation: appSettings,
            builder: (context, snapshot) {
              return MaterialApp(
                theme: ThemeData.light(),
                darkTheme: ThemeData.dark(),
                themeMode: appSettings.theme,
                home: const MyAwesomeApp(title: "Dark Theme Sample"),
              );
            });
      }),
    );
  }
}

class MyAwesomeApp extends StatefulWidget {
  const MyAwesomeApp({Key? key, required this.title}) : super(key: key);
  final String title;
  @override
  State<MyAwesomeApp> createState() => _MyAwesomeAppState();
}

class _MyAwesomeAppState extends State<MyAwesomeApp> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Consumer<Settings>(builder: (context, appSettings, _) {
          return Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: <Widget>[
              ToggleButtons(
                  children: const [
                    Text('Light'),
                    Text('Dark'),
                    Text('System'),
                  ],
                  constraints:
                      const BoxConstraints(minWidth: 80, minHeight: 40),
                  onPressed: (int index) {
                    print(index);
                    ThemeMode theme = ThemeMode.light;
                    switch (index) {
                      case 0:
                        theme = ThemeMode.light;
                        break;
                      case 1:
                        theme = ThemeMode.dark;
                        break;
                      case 2:
                        theme = ThemeMode.system;
                        break;
                    }
                    appSettings.theme = theme;
                  },
                  isSelected: [
                    appSettings.theme == ThemeMode.light,
                    appSettings.theme == ThemeMode.dark,
                    appSettings.theme == ThemeMode.system,
                  ]),
              Text(
                appSettings.theme == ThemeMode.dark ? 'Dark' : 'Light',
                style: Theme.of(context).textTheme.headlineMedium,
              ),
            ],
          );
        }),
      ),
    );
  }
}

class Settings extends ChangeNotifier {
  ThemeMode _theme = ThemeMode.light;

  ThemeMode get theme => _theme;

  set theme(ThemeMode theme) {
    _theme = theme;
    notifyListeners();
  }

  @override
  void notifyListeners() {
    super.notifyListeners();
  }
}