Exactly what I was looking for. Too many good themes to just pick one, so now I get to have 'em on a cycle!

Allows you to select a certain subset of themes or just all the ones you have installed, and either have it change every time you open the browser, or at a set interval. The default intervals are all in hours, but I set my custom one to 20 minutes.

Swaps with any installed theme that you've chosen through a checkmark menu,
Allows swapping to occur x hours, or if you select the Custom input, it'll allow you to change every x minute

Super useful, my only lament is that the icon for the add-on is the same as the general Addon settings icon