revert to a 'stock' version of the ROM first before applying the new theme
You don't wipe anything to reinstall the ROM, unless your theme has borked something, in which case you have to do a total wipe.
Right, with the following warning. Boot normally; don't just install the ROM over it. That way you can see if it worked or not. Then you can install the ROM. Better safe than sorry.
And make a backup of the new theme-free ROM if it boots just in case the new theme doesn't play nicely with it.