White background during app launch

There's another background attribute in your theme - windowBackground, and you implicitly set it to white by inheriting your style from @style/Theme.Sherlock.Light.DarkActionBar (notice the .Light. part in the name).

You can either use dark version of theme or set it explicitly by including windowBackground in your style definition:

<item name="android:windowBackground">@color/your_color</item>