I just tested this twice, with different variables. Then tested it on a copy of "Try Zorin".
It worked each time... I am not sure what is causing it not to on yours...
I wonder if you are testing it in a Flatpak application that is referencing the Flatpak Zorin themes- that is the only thing I can think of that may interfere.
The same method as above should work:
.selection-mode .titlebar:backdrop:not(headerbar), .titlebar:backdrop:not(headerbar), headerbar:backdrop { background-color: #1a2022; background-image: none; }
Replace #1a2022 with the color you want.



