title: Override Android background in custom style for one theme but not another
body: <p>I'm using two themes for my Android application, <code>AppTheme</code> and <code>AppThemeDark</code>. I've set up a custom button style for each in the theme definitions:</p>
  
  <pre><code>&lt;!-- Theme definitions --&gt;
  &lt;style name="AppTheme" parent="Theme.AppCompat.Light.DarkActionBar"&gt;
      ...
      &lt;item name="android:buttonStyle"&gt;@style/ButtonStyle&lt;/item&gt;
      ...
  &lt;/style&gt;
  
  &lt;style name="AppThemeDark" parent="Theme.AppCompat.DayNight.NoActionBar"&gt;
      ...
      &lt;item name="android:buttonStyle"&gt;@style/ButtonStyleDark&lt;/item&gt;
      ...
  &lt;/style&gt;
  
  &lt;!-- Button styles --&gt;
  &lt;style name="ButtonStyle" parent="@style/Widget.AppCompat.Button"&gt;
      &lt;item name="android:ellipsize"&gt;end&lt;/item&gt;
      &lt;item name="android:maxLines"&gt;2&lt;/item&gt;
  &lt;/style&gt;
  
  &lt;style name="ButtonStyleDark" parent="ButtonStyle"&gt;
      &lt;item name="android:background"&gt;@drawable/gray_button_bg&lt;/item&gt;
  &lt;/style&gt;
  </code></pre>
  
  <p>Note that my AppTheme button, ButtonStyle, doesn't override the background of <code>Widget.AppCompat.Button</code>, but AppThemeDark's button style inherits ButtonStyle and it changes the background.</p>
  
  <p>Now, I would like to extend that buttonStyle with a new custom style for special buttons, e.g. <code>PrimaryButton</code>. In the AppTheme case, I do not want to change the background of the button. In the AppThemeDark case, i <em>do</em> want to change the background of the button.</p>
  
  <p>I would like to define PrimaryButton and have it either inherit the background (in the case of AppTheme) or use a new background (AppThemeDark). Something like this:</p>
  
  <pre><code>&lt;style name="PrimaryButton"&gt;
      &lt;item name="android:background"&gt;?attr/drawablePrimaryButtonBackground&lt;/item&gt;
  &lt;/style&gt;
  </code></pre>
  
  <p>But as far as I can tell, there's no way to define an attribute in AppThemeDark as a new drawable and AppTheme as "inherit from parent". Setting to transparent obviously makes the button background in AppTheme transparent.</p>
  
  <p>Is my only option to figure out what drawable is being used for a background in Widget.AppCompat.Button and define it locally? </p>