Which themes are installed is specific to your brand of BB. What model BB are you using? There is a thread that gives instructions on how to "unlock" all the vendor oem themes. Give this a try.
Can I/how do I install just a theme? 8700c - already downloaded and installed 4.2 Desktop and Cingular's 4.1.0.377 and have the "Today" theme. Is there a way (without completely reloading) to get/activate the "Today Plus" theme.
...and/or how does one rearrange what is on the Today home screen?