From your home screen, press menu> preferences> Appearance Settings> Dock background> custom.
You might also want to use a transparent icon for your app drawer so the app drawer icon doesn't cover up the center of the dock. If you want the transparent icon but dont have one I'll post it.
Here ya go.
To set it you would long press the app drawer icon in you dock and select change icon. Then you would pick custom icon, and pick your transparent icon.