We do not have FTP access to free hosted WordPress.com blogs and we not able to upload third party themes.
The only themes you can use on a free hosted WordPress.com blog are those found here > Appearance > Themes and showcased here > http://wordpress.com/themes/ We cannot access and edit any templates underlying our themes. Only Staff can edit as every blog wearing the same theme is using the same template.
Unless you are prepared to pay for a VIP account, http://vip.wordpress.com/our-services/ there is no upgrade you can purchase that changes this at WordPress.COM.
To upload themes you need to hire a web host and set up a WordPress.ORG install. To clearly understand the differences between WordPress.COM free hosted blogs and WordPress>ORG installs for self hosting read this comparison > http://support.wordpress.com/com-vs-org/