You can use the right and left arrow keys.
No Jim, I am talking about moving between the three menus levels typically visible in TheaterView - the top menu, the submenu and the 'main' view window. The arrow keys are mainly used to navigate within a menu. TAB and alt-TAB can be used to cycle through the menus. I know up/down also works between the first two and the main view, but that breaks down if you are someway down a main view. Then, if you wish to change the main view and you want to use the arrow keys, you have to first scroll to the top of the view. TAB lets you jump straight to the other menus. The only problem is both TAB and alt-TAB cycle the same way though the menus which is not consistent with MC documentation and needlessly awkward.
I have heard it said that Theaterview is not even supposed to support TAB, but that fact is it does and it is very useful if, like many of us, you use a full AV remote like the Harmony.