change dark theme in flutter code example

Example 1: how to change background color in flutter theme

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData(
        canvasColor: Colors.green,
      ),
      home: Scaffold(
        appBar: AppBar(
          title: Text('Changing background color using theme'),
        ),
        body: Container(
        	child: Center(
            	child: Text('Some widget goes here.')
            )
        ),
      ),
    );
  }
}

Example 2: flutter dark theme

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