Every time I think I understand everything there is to know about CSS (I’m being very facetious), Heather Solomon blogs about something I had no idea about. Her latest trick is to use attribute selectors to change the formatting of specific types of Web Parts. This is another trick that is great for SharePoint but can certainly be used anywhere you are using CSS.
I’ll just send you straight to Heather’s post entitled Controlling Single Web Parts with CSS for the details. Heather’s explanation is, as always, clear and immediately usable.