theme.dart

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

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

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

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

class _MyAppState extends State<MyApp> {
  late final theme = CustomTheme();
  @override
  void initState() {
    theme.addListener(() => setState(() {}));
    super.initState();
  }

  @override
  void dispose() {
    theme.removeListener(() => setState(() {}));
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return ThemeInheritedWidget(
      theme: theme,
      child: Builder(
        builder: (context) {
          final _theme = ThemeInheritedWidget.of(context)!.theme;
          return MaterialApp(
            theme: ThemeData.light(),
            darkTheme: ThemeData.dark(),
            debugShowCheckedModeBanner: false,
            themeMode: _theme.mode,
            home: Home(),
          );
        },
      ),
    );
  }
}

class Home extends StatelessWidget {
  const Home({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    final _theme = ThemeInheritedWidget.of(context)!.theme;
    return Scaffold(
      appBar: AppBar(
        backgroundColor: _theme.myTheme.appBarColor,
        actions: [
          Switch.adaptive(
            value: _theme.darkMode,
            onChanged: (val) {
              if (!val) {
                _theme.mode = ThemeMode.light;
              } else {
                _theme.mode = ThemeMode.dark;
              }
            },
          ),
        ],
      ),
      backgroundColor: _theme.myTheme.backgroundColor,
    );
  }
}

class ThemeInheritedWidget extends InheritedWidget {
  ThemeInheritedWidget({
    Key? key,
    required this.child,
    required this.theme,
  }) : super(key: key, child: child);

  final Widget child;
  final CustomTheme theme;
  static ThemeInheritedWidget? of(BuildContext context) {
    return context.dependOnInheritedWidgetOfExactType<ThemeInheritedWidget>();
  }

  @override
  bool updateShouldNotify(ThemeInheritedWidget oldWidget) =>
      oldWidget.theme.mode != theme.mode;
}

class CustomTheme extends ChangeNotifier {
  ThemeMode _mode = ThemeMode.system;

  ThemeMode get mode => _mode;

  bool get darkMode => mode == ThemeMode.dark;

  set mode(ThemeMode mode) {
    if (mode == _mode) return null;
    _mode = mode;
    notifyListeners();
  }

  MyTheme get myTheme => darkMode ? MyTheme.dark() : MyTheme.light();
}
///TODO ADD Many properties to this class as you wish
class MyTheme {
  final Color backgroundColor;
  final Color appBarColor;
  MyTheme({required this.backgroundColor, required this.appBarColor});

  MyTheme.light()
      : backgroundColor = Colors.red,
        appBarColor = Colors.amber;
  MyTheme.dark()
      : backgroundColor = Colors.blueGrey,
        appBarColor = Colors.indigo;
}