Are you kidding? CSS editing cannot be used to install advertising codes on free hosted WordPress.com blogs. No blogger initiated advertising, retailing or reselling the work or products created or services provided by anyone other than yourself is allowed on free hosted wordpress.COM blogs. E-commerce transactions via shopping carts and the like cannot be conducted on free blogs from and being free hosted by wordpress.COM
The advertising exceptions noted in support documentation are for high traffic blogs that qualify for and are accepted into the Ad Control program, soon to be replaced with the WordAds program, and for extremely high traffic blogs that qualify and are accepted into the paid VIP hosting program. The WordAds program is only for blogs that are well established and have at least 20,000 page views monthly, as well as, meeting the criteria for acceptance into the program.
If you do require an ecommerce site, advertising and/or affiliate links you can hire a web host and get a free software install from http://wordpress.org Refer to > http://support.wordpress.com/com-vs-org/