I read in another thread here on blackberryforums.com that someone had contacted BlackBerry support about wanting to edit or change the default T-Mobile theme on their device. The response from support was that a new version of the Plazmic Content Developer's Kit would have a Theme utility that would allow users to create custom themes. No release date was provided though.
Interesting.... now that a link to 'Themes' exists on the Extras site at mobile.blackberry.com, I'm not so sure. (although it says 'coming soon' when you click on it). I guess those would likely be internally built themes as opposed to externally built themes. Perhaps you are correct.