answering stackoverflow question https://stackoverflow.com/questions/49164592/flutter-how-to-change-the-materialapp-theme-at-runtime

darkmode2.dart

import 'package:flutter/material.dart';

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

Settings appSettings = Settings();

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return AnimatedBuilder(
        animation: appSettings,
        builder: (context, snapshot) {
          return MaterialApp(
            theme: ThemeData.light(),
            darkTheme: ThemeData.dark(),
            themeMode: appSettings.getTheme,
            home: 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: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            const Text(
              'Drag the button to change the Theme',
            ),
            Switch(
                value: appSettings.getTheme == ThemeMode.dark,
                onChanged: (isDark) {
                  if (isDark) {
                    appSettings.setTheme(ThemeMode.dark);
                  } else {
                    appSettings.setTheme(ThemeMode.light);
                  }
                }),
            Text(
              appSettings.getTheme == ThemeMode.dark ? 'Dark' : 'Light',
              style: Theme.of(context).textTheme.headline4,
            ),
          ],
        ),
      ),
    );
  }
}

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

  ThemeMode get getTheme => theme;

  void setTheme(ThemeMode theme) {
    this.theme = theme;
    notifyListeners();
  }

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