The themes can be customized by editing their appearance, while they are not active, to see what they would look like.
But there is no way to save these customizations.
That forces customizations to be only setup permanently "after" the them has been activated. This forces the theme adaption process on to the visitor's experience (on-line).
If the themes can be customized and saved prior to going live, then they can be properly set up, and turned on like a switch.
The save feature should allow for saving multiple groups of settings for each theme.
This would also allow rapid side by side comparison.
Select two (or more?) theme customizations, and compare the two in a side by side view.
Possibly have a text box for entering notes on the comparison, and ability to rate features.