Why doesn't the ActionBar Drop Shadow work? Android

Question

I do not use ActionBarSherlock.

I added android: windowContentOverlayto to my .xml styles, but there is still no shadow.

styles.xml:

<style name="MyAppActionBarTheme" parent="android:Theme.Holo.Light">
<item name="android:windowContentOverlay">@drawable/drawer_shadow</item>
</style>

AndroidManifest:

<application
    android:allowBackup="true"
    android:icon="@drawable/ic_launcher"
    android:theme="@style/MyAppActionBarTheme" >

What did I miss?

Is it possible that I can add an ActionBar Shadow programmatically?