you can't upload a theme here. anywhere that you see wordpress themes, those are not designed with our blogs here in mind.
however, if you want to convert a theme into pure CSS, that can be done.
go to the Write page of your dashboard, and use the image uploder to upload any of the images.
next, open the style.css file, and replace the relative image links with links to the ones you uploaded, e.g.: replace
now you can copy the entire style.css file, and paste it into the Edit CSS page of your dashboard. odds are, it won't look right. but you should be able to rename the selectors to match.
you can ignore the PHP templates.
if this doesn't make sense to you, you probably want to just use some of the existing customCSS skins: http://en.forums.wordpress.com/topic.php?id=9415&page&replies=12