We at wordpress.com do not download blog templates from wordpress.org and load them into our blogs here as they run a different software than we do. Having said that it's important to warn you that although some themes at wordpress.org and wordpress.com may appear to be the same they are not. At wordpress.com we are on a shared multiuser blogging platform and we are not allowed to alter our templates for security reasons. http://faq.wordpress.com/2005/09/27/edit-my-templates/
Basically if you do not wish to use one of the themes made available at wordpress.com through your admin section ->Dashboard -> Presentation -> Themes then you have two options. The choice you make may depend on the differences between css customizing a wordpress.com theme or moving to self hosting or a web host and altering a free wordpress.org template. Hopefully this information below will be useful.
(1) The please read me first before posting pink sticky at the head of the forum outlines the differences between wordpress.com blogs and wordpress.org blogs which are on different code bases http://wordpress.com/forums/topic.php?id=3700&replies=1
(2) css upgrade of wordpress.com blog
http://wordpress.com/blog/2006/08/04/custom-css/ The 15$ is just to alter the appearance of your theme and not the underlying php code. Purchasing the upgrade entitles you to edit CSS on one blog for one year.
The CSS Editor does not let you edit any particular theme stylesheet; rather, it keeps the theme intact and lets you edit a brand new stylesheet. Because your custom CSS is loaded after the theme's CSS, your style rules can override or add to the existing rules.
You can use CSS to improve on any existing theme; you can define your own classes for use in posts; you can select the Sandbox theme and build on one of the Minimalist layouts or opt for no stylesheet and do it all yourself.