Originally Posted by Silverangel
So if the UI is showing up as an option in game, then the game is finding the SkinDefinition.xml file. If you see grey blocks after selecting that ui theme, then some or all images are missing. Some of these interfaces are very outdated, and some of those grey blocks might be due to outdated re-definitions over time. The best you can do is open the SkinDefinition.xml file in a text editor and ensure that every definition line is going to an image with the correct relative folder path.
Thanks for the tip. I was facing the same problem. It took some time, but I was able to edit the SkinDefinition.xml file in a text editor and now there are no grey boxes.