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 CSS upgrade, which will allow you to stylize the themes here, but it does not allow you to change the functionality. CSS is also themes specific so you cannot use the CSS from one theme with another theme. 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.