There are three themes that are currently active and working properly with this version of the system. Basically there is a black theme, a gray theme, and a light blue theme, and a few more.
Black theme - blackisblack
Gray theme - UNI
Light Blue Theme - ALT
To change your theme you must look at the very bottom of any page for the copyright and site info box. There you will see the current theme. If you click the theme link you will get a popup box.
The popup has the other themes just click the one you like. .. please do not click Simple as it is not for this version of the system.