We cannot add themes at wordpress.COM. We are limited to the 100+ (and growing) 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.
Themes here have to be modified to work within the restrictions here and to support the unique features here. Themes straight off the internet (which are for self-hosted blogs) will not work here.
CSS is also themes specific so you cannot use the CSS from one theme with another theme.
To use your theme, you would have to self-host a wordpress site on a third-party hosting service such as godaddy, bluehost, etc. using the software from wordpress.ORG. Figure about $10 to $25 per month all costs considered.