I found this thread in the Locked (and hidden) archives of the Features & Ideas forum:
 

TranceCanada
There's already a crap ton of things to change in the theme editor, but it would be nice if we could change the color of the synths in the synth rack, like the blue background for the names and the text color

-August 30, 2016
 
The text color was available but the blue background was never offered up for Sonar and was never found with Duckbar. it seemed to be hidden deep within the exe file. But today while poking around in BandLab's "Cakewalk" Theme Editor I noticed it was there.
 
Go to Theme - Browser - Synth Rack - Synth Strips. They're called "Synth Name Bezels".

Yes, that was added some time ago. There have been quite a few changes to themes since Aug 2016. The theme developer, Ben Stanton now works for BandLab.

We can change the meter colours on the Perfomance module now too