it's pretty simple
first, go to Recovery Console (hold R) and connect USB to Flash 0
Backup your entire Flash0 -- make a folder, and copy paste everything there -- this is so if you want to go back to normal, or if you royally fuck up (no need to worry about brick, just run a Pandora)
then head over here
http://consolespot.net/?network=xmbbrowse around, see if there's anything that catches your eye -
make sure its compatible with your firmware - the theme i'm using is
Organhttp://download.yousendit.com/783308F013729A02then, when you download your theme, go back to Flash0, and simply copy/paste to their respective folders, then boot up to XMB and hope you didnt fuck up