We cannot edit either themes or templates on free hosted wordpress.com blogs. The CSS editing upgrade does not allow for any changes in functionality at all. It's only for appearance changes and all CSS editing here is theme specific.
Since there is no official Staff support for the CSS upgrade, and precious little Volunteer support, I recommend that you don’t buy the upgrade unless you have at least moderate understanding of HTML and CSS because you must be prepared to do the work on your own.
If you are considering purchasing the upgrade, you can go to Appearance > Edit CSS. There wordpress has provided a preview function where you can try before you buy.
May I further suggest that you read this entry very carefully? > WordPress.com vs. WordPress.org: The Differences > http://support.wordpress.com/com-vs-org/