If you are talking about the blog linked to your username, 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 here, but it does not allow you to change the functionality. Since there is no official support for the CSS upgrade, do not buy the upgrade unless you have at least moderate understanding of CSS. 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.