There is nothing to fix, that's the way the author set the dock up. You may also see this on some zoomer docks as well. If the author has set a blank tile png in there to help shrink the icons down a bit to fit better on the dock, then you wont see the background in the preview area either. It's a bit unfortunate because you can't see what you are going to apply before you apply it...but it doesn't take much to apply then change if you don't like it. If you really want to see them, open the folder that has the docks and remove/rename the blanktile.png file.(That will screw up the setting for the dock though) But for tabs, you're stuck because a blank tab is what was designed.