Major CSS render issues

I am creating a new site and have it hosted at wordpress.com. I have the Custom CSS upgrade. However, whenever I add custom CSS it appears to render both the CSS I enter as well “competing” style sheet (from one of the defaults). The result is of course, jibberish.

So, I tried to revert to one of the free, basic themes. I tried Contept – and from looking at it in Firebug, it isn’t downloading the Contempt images at all, and its still showing remnants of my old CSS. Instead of the contempt CSS I get a blank CSS template with nothing in there except the comment /* mooooooo */.

Is there a way to hard reset all the CSS and start from scratch?