The blog linked to your username is an empty WordPress.COM blog wearing the Choco theme. All CSS editing on WordPress.COM blogs is theme specific. Our themes at WordPress.COM, even those with the same names as the WordPress.org themes, are not the same. WordPress.com and WordPress.ORG run on different software. Therefore as leonfrey is CSS editing MistyLook the editing he is doing is not applicable to any other theme. Please read this entry > http://support.wordpress.com/com-vs-org/
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. It does not allow you to remove footer links to the theme designer or to wordpress.com. Also note that CSS is theme specific, so you cannot use the CSS stylesheets from other themes with the themes here at wordpress.COM.
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.
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/