title: Drawable as styleable and how themes are selected
body: <p>In this code <a href="https://android.googlesource.com/platform/frameworks/base/+/android-7.1.1_r6/core/java/com/android/internal/widget/ToolbarWidgetWrapper.java#104" rel="nofollow noreferrer">ToolbarWidgetWrapper</a>
  I see that the <code>Drawable</code> is defined as styleable.<br>
  I tried to find what is that drawable and I found definitions such as:  </p>
  
  <pre><code>res/res/values/themes.xml:          &lt;item name="homeAsUpIndicator"&gt;@drawable/ic_ab_back_holo_dark&lt;/item&gt;
  res/res/values/themes_holo.xml:     &lt;item name="homeAsUpIndicator"&gt;@drawable/ic_ab_back_holo_dark&lt;/item&gt;
  res/res/values/themes_holo.xml:     &lt;item name="homeAsUpIndicator"&gt;@drawable/ic_ab_back_holo_light&lt;/item&gt;
  res/res/values/themes_holo.xml:     &lt;item name="homeAsUpIndicator"&gt;@drawable/ic_ab_back_holo_dark&lt;/item&gt;
  res/res/values/styles.xml:          &lt;item name="collapseIcon"&gt;?attr/homeAsUpIndicator&lt;/item&gt;
  res/res/values/themes_material.xml: &lt;item name="homeAsUpIndicator"&gt;@drawable/ic_ab_back_material&lt;/item&gt;
  res/res/values/themes_material.xml: &lt;item name="homeAsUpIndicator"&gt;@drawable/ic_ab_back_material&lt;/item&gt; 
  </code></pre>
  
  <p>It seems that this kind of definition ensures that the icon corresponds to the app's theme and I see the icons are completely different in each definition.
  Is that right? And if so, I was wondering how exactly the corresponding file is being selected? Does it depend only on some app configuration about the theme?<br>
  Is there a case that none of these would be selected?  </p>