For example, if you use the parent definition of Style , add only the android:popupMenuStyle attribute, as shown below:
<style name="Theme.Example" parent="@android:style/Theme.Holo.Light"> <item name="android:popupMenuStyle">@style/PopupMenu.Example</item> ....... ....... </style>
And override the android:popupMenuStyle attribute, specifying your style:
<style name="PopupMenu.Example" parent="@android:style/Widget.Holo.Light.ListPopupWindow"> <item name="android:popupBackground">@drawable/menu_dropdown_panel_example</item> </style>
@drawable/menu_dropdown_panel_example :

You can select an image similar to the above, or use the Color resource.
And this is for ActionBar by default. Not an ActionBarSherlock .; -)
ActionBar styles can be easily created using this cool website: http://jgilfelt.github.com/android-actionbarstylegenerator/ . I usually experiment a bit here before deciding on a style. In addition, it allows you to load styles and all the necessary resources. Just connect them to your application and you are good to go. :-)
UPDATED: I think I caused some confusion that led to the loss of shadow in your application. You may have replaced all <items> with the Style that I specified in the original sentence. Editing should fix this.
UPDATE 2:
Use these image resources instead of the @android:color/white value you are currently using. They are in XHDPI, HDPI, and MDPI order. Save them and use them in your style definition.



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