We cannot add themes at wordpress.COM. We are limited to the themes found in the dashboard under appearance > themes. Themes found on the internet are for self-hosted wordpress blogs using the software from wordpress.ORG.
If you have CSS editing experience, you can get the paid custom CSS upgrade, which will allow you to stylize the themes already here, but it does not allow you to change the functionality. Since there is no official support for the CSS upgrade, and few here in the forums with much CSS experience, you should figure that you are on your own. Before buying the upgrade, you can go to appearance > edit CSS and wordpress has provided a preview function where you can try out changes before you buy the upgrade.
Also note that CSS is theme specific, so you cannot use the CSS from other themes with the themes here at wordpress.COM.