title: the value of "?attr/colorPrimary"
body: <p>I'm learning to use the <strong>Toolbar</strong> widget.<br>
  while following the guide on <code>developer.android.com</code>, I find this </p>
  
  <pre><code>android:background="?attr/colorPrimary"
  </code></pre>
  
  <p>when I go to the definition of <strong>"?attr/colorPrimary"</strong>, I only find </p>
  
  <pre><code>&lt;attr format="color" name="colorPrimary"/&gt;
  </code></pre>
  
  <p>so how can I know the value of <strong>"?attr/colorPrimary"</strong></p>