Nice article by Bill Konrad over at socialsnippets.com that covers the default facebook css styling:

http://www.socialsnippets.com/snippets/how-to-style-an-application-like-facebook.aspx

“…explore how we can style our Facebook application to look and feel like an extension of the Facebook platform. We will examine colors, fonts, dimensions and a few common design patterns utlized by the Facebook platform in our quest to fit in with the crowd. ”