The only themes we can use on WordPress.com blogs like your blog are licensed and adapted to run on our multiuser software the themes are found here http://wordpress.com/themes/
There is no FTP access to free hosted WordPress.com blogs for security reasons and we cannot:
upload any third party themes,
create child themes,
create our own themes,
create own own layouts,
or use stylesheets from other themes.
There is no upgrade you can purchase that changes that. WordPress.org offers free software that you can install on a web server. You can create themes, upload and install themes and plugins, run ads, and edit the database, etc. If you want FTP access and to install plugins or third party themes into a site then you will have to hire a web host http://wordpress.org/hosting/ and set up your own WordPress.org install.