Alright, I did this, and edited the BodyCache.bin by renaming it to body_LZ.bin and modifying it with Every File Explorer (changed the images). I saved, and changed it back to BodyCache.bin, and deleted the ThemeManage. I then copied the caches from sd to extdata using the CIA version of the tool, and rebooted. After running GW (to get back into the system menu with themes) The theme was blank. After selecting Sonic Boom (the theme I modified?) It loaded the Sonic Boom Theme normally.
Did I do something wrong?
It uses NinjHAX to change themes to custom ones, found on the page before this one.