I agree with everyone above. I used to have all the themes installed but since the memory issues started about a year ago, I no longer keep all those themes. I simply edit the .alx to add all vendor locked themes but then just remove the ones I'm not using from the java folder. Then if I want to change I simply copy and past that .cod back into the java folder and run app loader.
I dont usually add 3rd party themes as they have been known to cause strange problems on the device. Many don't but, again, with the new BB's you need all the memory you can get.