Is there such a possibility? Can it be developed?
Per-user custom CSS
It would be cool if each user was able to define their own custom CSS rules, which would override (or not) the forum-wide CSS rules. Of course, this feature could be disabled in the ACP
Users can use a browser plugin like Stylish to do this themselves if they desire. Of course, I'm sure it's fairly trivial to make a plugin that would do this.