928droid makes themes. a theme is just a visual change to your system framework (i.e. black status bar, different font, etc), whereas a rom is actually a re-worked version of the OS. some themes are designed to work with different roms, some will work with stock roms. usually it is specified wherever you downloaded the theme from which roms the theme is compatible with.
you dont have to wipe when applying a theme only (i never had, and have never had an issue), BUT always make a nandroid backup before applying any theme to your phone, just in case the theme screws something up, or if you find you just dont like it
now one more very important question for you: after you rooted, did you deodex your system files? if not, DO NOT apply any themes until you are deodexed, as it will mess up your phone
btw, also moving to rom/hacks forum