title: Yii2 theme integration
body: <p>I have installed Yii2 advanced application, and now I want to change backend theme. 
  How can I do this?
  Is there any file where I need to tell Yii2 that use my custom theme? 
  I established my theme under <code>backend/web/themes/mytheme</code>.
  I just replaced this code in <code>advanced/backend/config/main.php</code>, but nothing happened!</p>
  
  <pre><code> 'view' =&gt; [
    'theme' =&gt; [
         'pathMap' =&gt; ['@app/views' =&gt; '@app/themes/mytheme'],
         'baseUrl' =&gt; '@web/themes/mytheme',
      ],
    ], 
  </code></pre>
  
  <p>Then I replaced this code under <code>common/config/main.php</code> but nothing changed!</p>