title: Can't change flutter theme color
body: <p>I need to change the flutter theme color, but when I tried then run my app it's show nothing to change. I don't know why.</p>
  <p>I have tried this way:</p>
  <pre><code>import 'package:flutter/material.dart';
  
  import 'pages/home_page.dart';
  
  void main() {
    runApp(MyApp());
  }
  
  class MyApp extends StatelessWidget {
    @override
    Widget build(BuildContext context) {
      return MaterialApp(
        home: HomePage(),
        themeMode: ThemeMode.dark,
        theme: ThemeData(primarySwatch: Colors.deepPurple),
        darkTheme: ThemeData(
            brightness: Brightness.dark, primarySwatch: Colors.deepPurple),
      );
    }
  }
  
  </code></pre>
  <p>when I change like this way, then still show me default blue color.</p>
  <p>flutter version: <code>2.2.1</code></p>
  <p>Any suggestion please.</p>