There are no themes available other than what the individual carriers have created with RIM. You cannot change the themes themseleves nor are there any other themes available. What Jibi has posted in his sticky is the best out there. He did a great job with them. There is no better.
There is a 'Themes' on the BlackBerry Extras site at mobile.blackberry.com, but it says 'coming soon!'. I'm assuming there are plans to put up additional themes on the Extras site sometime in the future.