title: Change md-checkbox color
body: <p>So it looks I can change the background-color of an empty <code>md-checkbox</code> by doing :</p>
  
  <pre><code>md-checkbox .md-icon {
  background: red;
  }
  </code></pre>
  
  <p>But I can't manage to change the background of a checked <code>md-checkbox</code>.
  I tried  to play with <code>:checked</code>, <code>::before</code>, <code>::after</code>... But didn't succeed.</p>
  
  <p>How should I proceed ?</p>