Throwing out things just as I think them.
This is something present in wikimedia, and I find it useful sometimes: have a custom style sheet and javascript.
In other words each user could be able to style the forum the way he wants (mostly of course) and just for his personal use, not something intended to customize the profile page like ultimate profile, something like...let's say I don't like the color of the active button in the menu, I just add a definition to my personal css to change it to something different, that's all.
GreaseMonkey. Not default feature.
I don't even know what GreasMonkey is (and don't care, don't waste time linking it :P).
FF add-on. Allows people to write their own custom shiz for web pages.
Anyway, seriously, forget personal css as a default feature. That way lies madness.
Well...not sure why it would be more crazy than greasmonkey, though...you are an admin so I believe you. :P
/me goes back writing his mod.
It's not, but the good thing about GreaseMonkey is that people inflict it on themselves, and it's nothing to do with the site software. I like that. :D
No debugging. No cross-browser shiz. No complaints if they bork their own css. No performance hit for people who are happy with the standard css. Etc, etc, etc, etc.