final_code.dart

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

final darkTheme = ThemeData(
  primarySwatch: Colors.grey,
  primaryColor: Colors.black,
  brightness: Brightness.dark,
  backgroundColor: const Color(0xFF212121),
  accentColor: Colors.white,
  accentIconTheme: IconThemeData(color: Colors.black),
  dividerColor: Colors.black12,
);

final lightTheme = ThemeData(
  primarySwatch: Colors.grey,
  primaryColor: Colors.white,
  brightness: Brightness.light,
  backgroundColor: const Color(0xFFE5E5E5),
  accentColor: Colors.black,
  accentIconTheme: IconThemeData(color: Colors.white),
  dividerColor: Colors.white54,
);

class ThemeNotifier with ChangeNotifier {
  ThemeData _themeData;

  ThemeNotifier(this._themeData);

  getTheme() => _themeData;

  setTheme(ThemeData themeData) async {
    _themeData = themeData;
    notifyListeners();
  }
}

void main() => runApp(ChangeNotifierProvider<ThemeNotifier>(
      create: (_) => ThemeNotifier(lightTheme),
      child: MyApp(),
    ));

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    final themeNotifier = Provider.of<ThemeNotifier>(context);
    return MaterialApp(
      title: 'Dark Mode Example',
      theme: themeNotifier.getTheme(),
      debugShowCheckedModeBanner: false,
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  String mode = 'Light';
  bool isDark = false;

  @override
  Widget build(BuildContext context) {
    final themeNotifier = Provider.of<ThemeNotifier>(context);

    return Scaffold(
        body: Stack(
      children: <Widget>[
        Center(child: Text('Mode is: ' + mode)),
        Align(
            alignment: Alignment.topLeft,
            child: Padding(
              padding: const EdgeInsets.all(40.0),
              child: Switch(
                  value: isDark,
                  onChanged: (bool newVal) {
                    setState(() {
                      isDark = newVal;
                    });
                    if (isDark) {
                      mode = 'Dark';
                      themeNotifier.setTheme(darkTheme);
                    } else {
                      mode = 'Light';
                      themeNotifier.setTheme(lightTheme);
                    }
                  }),
            ))
      ],
    ));
  }
}