I'm using Xcode 13.4.1 (13F100) and have been stumped trying to figure out why toggling dark mode via Shift+CMD+A or Features -> Toggle Appearance was not working. Unfortunately, clearing derived data did not work and I had not yet ventured into deleting much more. I did, however, discover that within the Debug Tray toolbar at the bottom of Xcode, there is an Environment Overrides icon that opens a panel. If I toggle the Appearance switch, I'm able to freely switch between light & dark mode in the simulator. However, this setting does not re-enable the hot key or the menu option, but it works! Hope this helps someone else like me who was going insane trying to figure this out