I'm sorry but that's not possible on a free hosted WordPress.com blog. There is no FTP access to free hosted WordPress.COM blogs and we cannot install individual plugins or themes into them. Those found on the internet are for WordPress.ORG installs which run on different software.
On free hosted wordpress.COM blogs we can only use the themes which have been coded to run on this multi-user software and made available to us here. > Appearance > Themes and no others. We cannot edit either themes or templates on free hosted wordpress.COM blogs. > http://en.support.wordpress.com/themes/editing-themes/ All blogs on thei multiuser blogging platform wearing the same theme are using the same template and only Staff can access those files and edit. Every edit they make affects all blogs wearing the same theme.
You cannot build or use your own custom theme built from scratch at WordPress.com. The only themes we can use are those found here > Appearance > Themes
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/
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.
If you have CSS editing experience the upgrade will allow you to stylize the appearance of themes you find here > Appearance > Themes, but will 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 at WordPress.COM. http://en.support.wordpress.com/custom-design/custom-css/
See also > WordPress.com vs. WordPress.org: The Differences > http://support.wordpress.com/com-vs-org/