But regardless! As I mention in another place on this thread: The point of this project is the blunt comparison to The Onion -- the copying of the name, look, and feel of a well known and well recognized satirical publication is actually a vital part of the statement about our world and the media.
But, it should never be a step (baby or otherwise) to completely take someone else's work. It's one thing to take cues from them. It's another thing to literally take their code without permission, even if it is "only CSS". That is really, really poor form from the start.
If you don't think so, then try asking The Onion if they mind. Then you can have a clear conscience.
But, if you are unwilling to even ask, well, then you really don't have to. You already know it's not cool.
BTW, good luck with it.
Edit: its only a bad idea if you care about the integrity of your site, for something like this it probably doesn't matter.