theme.dart

import 'dart:async';

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return StreamBuilder(
      stream: bloc.darkThemeEnabled,
      initialData: false,
      builder: (context, snapshot) => MaterialApp(
          debugShowCheckedModeBanner: false,
          theme: snapshot.data ? ThemeData.dark() : ThemeData.light(),
          home: Scaffold(
            appBar: AppBar(
              title: Text("Dynamic Theme"),
            ),
            body: Center(
              child: Column(
                crossAxisAlignment: CrossAxisAlignment.center,
                mainAxisAlignment: MainAxisAlignment.center,
                children: <Widget>[
                  ClipRRect(
                    borderRadius: BorderRadius.all(Radius.circular(70)),
                    child: new Image.network(
                      "https://www.gstatic.com/tv/thumb/persons/589228/589228_v9_ba.jpg",
                      width: 150,
                      height: 300,
                    ),
                  ),
                  Padding(
                    padding: const EdgeInsets.only(left: 10.0, right: 6.0),
                    child: new Text(
                      "Hello Mark Zukerberg!!!!!!",
                      style: TextStyle(
                          fontSize: 15.0, fontWeight: FontWeight.bold),
                    ),
                  ),
                  Switch(value: snapshot.data, onChanged: bloc.changeTheme),
                ],
              ),
            ),
          )),
    );
  }
}

class Bloc {
  final _themeController = StreamController<bool>();
  get changeTheme => _themeController.sink.add;
  get darkThemeEnabled => _themeController.stream;
}

final bloc = Bloc();