That’s not possible. WordPress.COM is a multiuser blogging platform and we have no FTP access to ou rblogs so we cannot upload any themes. We run of idifferent software from that at wordpress.ORG.
Please see here for the themes we can use > Appearance > Themes
Live demo site is here > http://theme.wordpress.com/
Clarification here > http://support.wordpress.com/com-vs-org/