title: android toolbar popupTheme vs theme
body: <p>Often I see this declaration of Toolbar in layout files:</p>
  
  <pre><code>&lt;android.support.v7.widget.Toolbar
      xmlns:android="http://schemas.android.com/apk/res/android"
      android:layout_width="match_parent"
      android:layout_height="?attr/actionBarSize"
      app:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar"
      app:popupTheme="@style/ThemeOverlay.AppCompat.Light"&gt;
  &lt;/android.support.v7.widget.Toolbar&gt;
  </code></pre>
  
  <p>Why are there two attributes relating theming: theme and popupTheme?<br/><br/>
  What are the purposes of each of them?</p>