Theme.AppCompat is used to set the global theme for the entire app. ThemeOverlay.AppCompat is used to override (or "overlay") that theme for specific views, especially the Toolbar.
Let's look at an example for why this is necessary.
App themes with an ActionBar
The ActionBar is normally shown in an app. I can choose it's color by setting the colorPrimary value. However, changing the theme changes the color of the text on the ActionBar.
<style name="AppTheme" parent="Theme.AppCompat">
<item name="colorPrimary">@color/colorPrimary</item>
<item name="colorPrimaryDark">@color/colorPrimaryDark</item>
<item name="colorAccent">@color/colorAccent</item>
</style>
![enter image description here]()
Since my primary color is dark blue, I should probably use one of the themes that uses a light text color in the action bar because the black text is hard to read.
Hiding the ActionBar and using a Toolbar
The whole point of using Theme.AppCompat rather than Theme.Material is so that we can allow older versions of Android to use our material design theme. The problem is that older versions of Android don't support the ActionBar. Thus, the documentation recommends hiding the ActionBar and adding a Toolbar to your layout. To hide the ActionBar we have to use one of the NoActionBar themes. The following images show the Toolbar with the ActionBar hidden.
![enter image description here]()
But what if I want something like a Light theme with a DarkActionBar? Since I have to use NoActionBar, that isn't an option.
Overriding the App Theme
Here is where ThemeOverlay comes in. I can specify the Dark ActionBar theme in my Toolbar xml layout.
<android.support.v7.widget.Toolbar
...
android:background="?attr/colorPrimary"
android:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar" />
This finally allows us to have the effect we want. The Dark.ActionBar theme overlays the Light app theme for this particular occasion.
![enter image description here]()
- App Theme:
Theme.AppCompat.Light.NoActionBar
- Toolbar Theme:
ThemeOverlay.AppCompat.Dark.ActionBar
If you wanted the popup menu to be light you could add this:
app:popupTheme="@style/ThemeOverlay.AppCompat.Light"
Further Study
I learned this through experimentation and through reading the following articles.
