Is there a way to make the rel=nofollow part of a CSS Style?

No, because it's not a style and has nothing to do with the presentation of a page.

"Sort of." There are two ways to do it (I use the latter):

1) Use Javascript to detect all external links and apply a CSS class to them.

2) Apply a CSS class (I use .no-follow-link) to each nofollowed link.

Neither is technically the right answer because, as gringo said, link relationships don't have anything to do with page presentation. It's a good thought, though. You're not the only one that has had it (trust me on this one.)