In the MCT folder you will find additional themes for Canada, Australia, South Africa, and Great Britain.
You can go in and grab just the Wallpapers…
Or go into the Theme folder double click on the the thumbnail to add it.
Here you can see I added all of them and now they will be available under the My Themes section for when you want to change them around.
This is a neat little trick that will let you customize Windows 7 with hidden themes already available. Also if you want to get more themes head over to the Microsoft site.