Go to: http://live.playstation.com/ login to your PSN account, watch the Call of Duty trailer, claim your prize and you'll get a redemption code for the theme. US-only.
Not sure, but themes should be available on the system once installed, no matter on which account you're logged in. So you can just get it with an US account and use it on your regular non-US account. Please correct me if i'm wrong.