Holy fuck do things get off track, took me forever to find who was looking at how to change a pattern category and where it was posted.
Dr House I think I found it for you. I can't test for sure as patterns make my game crash. I suspect crap video card. Anyway.
Download Postal - See previous link
Open Pattern in Postal
Double click the xml - Pattern List
Look below at xml
See the line... <category name="Theme">
Change what is in quotes to what you want.
Commit
File / Save As
It should work. I also would guess proper capitalization is required. I don't know the exact names of all the categories, it will take a bit more digging.