Форумы / Cotonti / Support / Custom CSS Profile

You have to put it into the header.

Something like this as CSS-file /datas/users/name_[userID].css

The name before the ID should be fixed, otherwise it will be hard to do.
You also need to make sure that there should be a default one loaded before the custom one.
Otherwise you have no CSS-file at all if user hasn't uploaded one.

I guess you need to do this via a plugin. Because USERID is not a standard tag.

This is just an idea. Not sure it will work.