Flutter How to set onPressed to change ThemeData?

Question

I want to change the theme to dark theme & return to light theme with this button:

IconButton(

       icon: Icon(
        Icons.brightness,
        ),
       onPressed: () {
       setState(() {
       // Change to dark theme?
        } else {
       //Change it back to default light theme //How?
  }
}
);

Main.dart

MaterialApp(

  theme: ThemeData(
  ),
  darkTheme: ThemeData(
    brightness: Brightness.dark,
  ),

Answer 1

first is set to condition in your material theme (may be you define this is in main class)

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

StreamController<bool> setTheme = StreamController();

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return StreamBuilder<bool>(
        initialData: true,
        stream: setTheme.stream,
        builder: (context, snapshot) {
          return MaterialApp(
              theme: snapshot.data ? ThemeData.light() : ThemeData.dark(),
              debugShowCheckedModeBanner: false,
              home: Scaffold(
                  appBar: AppBar(title: Text("Your Theme")),
                  body: YourButtonPage()));/*Change this name with Your class*/
        });
  }
}

class YourButtonPage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Padding(
        padding: const EdgeInsets.all(16.0),
        child: Center(
            child: Row(
                mainAxisAlignment: MainAxisAlignment.center,
                children: <Widget>[
              IconButton(
                  icon: Icon(
                    Icons.brightness_low,
                    size: 21,
                  ),
                  alignment: Alignment.center,
                  onPressed: () {
                    setTheme.add(true);
                  }),
              IconButton(
                  icon: Icon(
                    Icons.brightness_1,
                    size: 21,
                  ),
                  alignment: Alignment.center,
                  onPressed: () {
                    setTheme.add(false);
                  }),
            ])));
  }
}