Here's more on CSS editing. If you have CSS editing experience, you can get the paid custom CSS upgrade and renew it annually. It will allow you to stylize the appearance of themes you find here > Appearance > Themes, but it does not allow you to change the functionality, by editing the underlying template.
Note that CSS is theme specific, so you cannot use the CSS stylesheets from other themes with the themes here at wordpress.COM.
Also note that you are not allowed to remove or hide the link to the web designer who created the theme nor the "blog at wordpress.com" link from the footer of the theme.
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.
If you do purchase the upgrade be sure to read this article before starting your editing > http://csswiz.wordpress.com/2009/10/15/if-you-have-the-wp-com-css-upgrade/