custom CSS

Need this for a customer (wanna use b2 + custom styling) and I'm too lazy for using diff patches against our standard CSS files..

idea: a user should be able to add thier own CSS declarations without modifying one of our files (makes an upgrade much easier)
Eman suggested a db base solution, however I think this is by far the easiest solution as it works per variant (that's a bit complicated if we place the custom css in the database)

Not sure if we should make it optional via $modSettings['load_custom_css']
could prevent an additional file_exists for those who don't use that "feature".