Probably not fully automatic, but create an extra "warning points" category. Every time you see a post telling you what a mean poopyhead you are, you give them the special warning point.
Have somebody create a plugin that links to and updates your sig based on the points count, and Bob's your uncle.
That's all assuming you can have that extra warning category.
As for the forums: I've always been more of a fan of white text on a dark background. It's a bit easier on the eyes for me.