the value of "?attr/colorPrimary"
<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>