title: Transparent background for button
body: <p>I need to set transparent background for my button, but only background, not all button.
  I've tried:</p>
  
  <pre><code>android:background=""
  android:background="null"
  </code></pre>
  
  <p>and background changed on transparent, but i've got an error:</p>
  
  <pre><code>Error: String types not allowed (at 'background' with value '').
  </code></pre>