Also note that here at WordPress.com we are limited to using only the theme we find here > Appearance > Themes
There is no FTP access to free hosted WordPress.COM blogs and we cannot install individual plugins or any themes into them The themes found on the internet are for WordPress.ORG installs which run on different software.
We can purchase a custom design upgrade provided we have CSS editing skills but you cannot build or use your own custom theme built from scratch at WordPress.com. This is a multiuser blogging platform. All blogs wearing the same theme are using the same underlying template and we cannot access the underlying files and edit templates or themes. http://en.support.wordpress.com/themes/editing-themes/
If you have CSS editing experience the custom design upgrade will allow you to stylize the appearance of themes you find here > Appearance > Themes
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 at WordPress.COM. http://en.support.wordpress.com/custom-design/custom-css/
There are two free themes, Sandbox and Toolbox designed to allow maximum flexibility for CSS editing and creating new “skins”. Toolbox is more modern and includes a lot of support for CSS3 and HTML5, so it is really only limited by your skills and knowledge.