how do you apply a custom theme/background on wordpress.com?

Selena asked:


i have a blog and i would like to add a theme on it.
the site is http://www.thecutestblogontheblock.com/free/wordpress-themes.html?start=10
and there are alot of cute wordpress backgrounds but me as a internet blog theme applying newb..
i have NO IDEA
on how to apply that onto the blog..
help?!