1. You shouldn't apply any themes made for 1.6, 2.1 or 2.2. They have different file names and locations for the images. Not many different, but enough to make your phone act up if you do apply it.
2. Never apply a theme that is made for a specific phone model. What I mean by this is if you took a theme made only for Backflip and applied it, you might be ok if it doesn't include services.jar (clock color). Reason being is that services.jar is rom specific.
3. You may be able to try to apply themes for CLIQ or Backflip to the CLIQ XT as they are all AOSP 1.5 w/MotoBlur. If you don't see something changed or you are getting FC's or other errors, then it needs to be tweaked before it will work.
Themeing is very tedious but simple once you get the hang of it if you decide to port or create one.