New preference: "user stylesheet", with a linky to the original Z one for people to take a copy of and customise.

Check box, put URL for custom stylesheet, apply when ready.

Even better would be the ability for a user to upload their own stylesheet; make a limit of, say, 1KB max to avoid people gaming it by creating multiple users and then doing some kind of weird chimaeran HTTP-with-basic-auth RAIZIWTUA thing to store monkey porn a la Google Mail.