If you want to remove the shadow below the ActionBar (for example in the AppCompat.Light theme), first create a style:

<style name="ThemeActionBarWithoutShadow"
    parent="@style/Widget.AppCompat.Light.ActionBar.Solid.Inverse">
  <item name="elevation">0dp</item>
  <!--<item name="android:elevation">0dp</item>-->
</style>

And then reference it from within a theme:

<style name="ThemeDefault" parent="Theme.AppCompat.Light">
  <item name="colorPrimary">#F2E49F</item>
  <item name="colorPrimaryDark">#F2E49F</item>
  <item name="actionBarStyle">@style/ThemeActionBarWithoutShadow</item>
</style>