I didn't find the answer to this problem on this forum, but i've found it elsewhere. Credit for my knowledge goes here:
MavROM 4.5 Theme Issue - Android Forums at AndroidCentral.com
To install a theme:
1) Reboot the phone into safestrap
2) Install .zip from SD card (internal sd)
3) Navigate to /Maverick/Themes/
4) You'll find the zip for the theme there. Select it, and install it.
I'm guessing the themes don't install via mav toolbox because of some kind of miscommunication between safestrap and the rom. You can do it this way, manually.