Since there is no official Staff support for the CSS upgrade, and precious little Volunteer support, I recommend that you consult tutorials found here > http://en.support.wordpress.com/custom-css/#css-help and read this article > consult tutorials found here > http://en.support.wordpress.com/custom-css/#css-help while waiting for a Volunteer to help you.
Here at wordpress.com all CSS editing is theme specific and we cannot use the CSS stylesheets from other themes. Provideing the URL for the blog in question so the Volunteers can see what you have done, and become aware of which theme you are editing would be a good thing to do. :)