Experimenting with Darkmode and I am having trouble setting the background color for the bevel button. The button has an icon but I have tried without the icon and can’t seem to make any change. Suggestions?
Unfortunately, this is a known issue.
Check out this thread for a replacement https://forum.xojo.com/55635-bevelbuttonreplacement-class
I hope that helps. Good luck 
Because:
https://developer.apple.com/design/human-interface-guidelines/macos/buttons/bevel-buttons/
This affected me as well.