Nilen Patel Inc.

Tutorial and code of theme in flutter. Copy and paste the below code as per your requirements.

main.dart

import 'package:flutter/material.dart';
import 'detail.dart';
import 'theme_changer.dart';

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

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return ThemeBuilder(
      defaultBrightness: Brightness.dark,
      builder: (context, _brightness) {
        return MaterialApp(
          debugShowCheckedModeBanner: false,
          title: 'Flutter Demo',
          theme: ThemeData(primarySwatch: Colors.blue, brightness: _brightness),
          home: MyHomePage(title: 'Flutter Theme Demo'),
        );
      },
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key? key, required this.title}) : super(key: key);
  final String title;

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

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  void _incrementCounter() {
    ThemeBuilder.of(context)?.changeTheme();
    setState(() {
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            RaisedButton(
              onPressed: () {
                Navigator.push(context,
                    MaterialPageRoute(builder: (context) => DetailPage()));
              },
              child: Text('Go to Detail'),
            ),
            Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.headline4,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }
}

theme_changer.dart

import 'package:flutter/material.dart';

class ThemeBuilder extends StatefulWidget {
  final Widget Function(BuildContext context, Brightness brightness) builder;
  final Brightness defaultBrightness;

  ThemeBuilder({required this.builder, required this.defaultBrightness});

  @override
  _ThemeBuilderState createState() => _ThemeBuilderState();

  static _ThemeBuilderState? of(BuildContext context)
  {
    return context.findAncestorStateOfType<_ThemeBuilderState>();
  }
}

class _ThemeBuilderState extends State<ThemeBuilder> {
  late Brightness _brightness;

  @override
  void initState() {
    super.initState();
    _brightness = widget.defaultBrightness;

    if(mounted)
    setState(() {
      
    });
  }

  void changeTheme() {
    setState(() {
      _brightness =
          _brightness == Brightness.dark ? Brightness.light : Brightness.dark;
    });
  }

  Brightness getCurrentTheme()
  {
    return _brightness;
  }

  @override
  Widget build(BuildContext context) {
    return widget.builder(context, _brightness);
  }
}

detail.dart

import 'package:flutter/material.dart';

class DetailPage extends StatefulWidget {
  @override
  _DetailPageState createState() => _DetailPageState();
}

class _DetailPageState extends State<DetailPage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Detail Page'),
      ),
      body: Center(
        child: Text('Welcome to Detail Page!'),
      ),
    );
  }
}