Currently there's the ability to define a background image.  But the interface elements are fixed colors, which often don't work well with different backgrounds.

Give the ability to select different color schemes for the interface.  Define maybe 8 to 12 standard schemes to choose from, plus allow users to define their own in a simple text file.

If many of the ui elements consist of images (rather than being rendered graphically), which is how they appear to be implemented, this ability should also permit redefining the images used for icons, buttons, etc.  I'd keep it simple, so that image sizes must be the same as the originals (no sense in creating layout issues).  This would give users a simple, yet fairly complete means of skinning the interface.