How do I revert to the original CSS of a theme on WordPress.com?

Hi there.

I’ve got a couple of years experience with CSS, but I’ve been tinkering with WordPress.com for just three days, so I have some embarrassingly basic questions. Like this one:

How do I revert to the default styles of the themes on WordPress.com? I’ve altered one a bit, but the changes seem to be imprinted on each theme I now load onto my blog. What I want is to load one of the themes right onto my blog, as-is, and then tinker with the CSS of that theme, rather than confusingly sorting out changes I made to a previous theme. Can you help?

Cheers,
Will