CSS is of course the "styling" part of your site, to help separate content and presentation. Search engines like for you to use CSS, as it makes the HTML lighter and cleaner and the engines don't get bogged down in presentation and styling stuff.
There are CSS galleries to which you can post links to your site in order to show off your skills. If you are a site developer, this sounds like a legitimate thing to do, but some site owners seem to be abusing this by submitting links to their site as just another backlink to generate rankings (i.e., they're not really doing it to show off their skills).
I predict that Google will soon start ignoring CSS gallery links back to sites.