Noa,
I have some bad news for you. On March 1st, Facebook announced a new Comment system. This new Comment system does not allow the inclusion of CSS files anymore. Originally, when we read the post, we thought that for users of the 'older' method, they would still be allowed to include CSS files. Unfortunately, that appears to not be the case, and there will be no way to include external CSS files going foward.
All of the changes were supposed to go into effect at the end of April, but we're already seeing limited (or no) use of external CSS files being a part of the comment box. There's nothing we can do about this, I'm sorry to say.
If you'd like to read the post, please see it below. Additionally, if you search for "css", you'll see another user complaining of the same issue. A Facebook moderator does say that they're looking into ways to have an option for a dark background, but no timeline is given.
developers.facebook.com/blog/post/472
I hope this helps, and good luck!