Excellent post from John! However, it made my Sprint Pearl 8130 crash but I did find a quick solution and would like to share it. Really, follow John's directions but when you get to the part about removing the vendorID tag from blackberry.alx, do this instead:
Instead of removing the vendorID tag, add your vendorID to the entry already there. Use a pipe (|) to separate it from the existing entry.
For example: _vendorID="104|102"
This allows Sprint and ATT to use that particular theme. So I just went down the list and added Sprint to the themes I wanted. Now for the 8130, ** DO NOT ** add your vendorID to the T-Mobile themes as this makes it crash. I suspect it's the myFaves.cod that's the culprit. I guess you could edit out the myFaves.cod from the T-Mobile theme entry, but it's easier to just not put your vendorID there. Anyway, now that you've added your vendorID to the themes you want, only those themes will get installed when your run the App Loader. No need to move *.cod theme files into a 'removed modules' folder as John directs. However if there are other *.cod files you'd like to remove, use John's method. That's it! Pretty simple and thanks to JohnClark for discovering this in the first place.
Again, I only know this works for my Sprint Pearl 8130. BTW, I've got the red Pearl and the Vodaphone interfaces (red and green) go perfectly with it. Looks slick and pro-like, much better than Sprint's. Good luck! -daver