I'm sorry but that's can can do on a wordpress.com free hosted blog. There is not FTP access to wordpress.com free hosted blogs. We cannot add themes at wordpress.COM, we are limited to the themes found in the dashboard under Appearance > Themes. http://en.support.wordpress.com/themes/adding-new-themes/
Also note that CSS is theme specific, so you cannot use the CSS stylesheets from other themes with the themes here at wordpress.COM so you cannot use the CSS stylesheets from other themes with the themes here at wordpress.COM.
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. 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/ There is no official Staff support for the CSS upgrade, and precious little Volunteer support, please be patient and consult tutorials found here > http://en.support.wordpress.com/custom-css/#css-help while waiting for a Volunteer to help you. :)