In order for Umbra to update system settings, including the system
appearance and desktop wallpaper, we need to request system event
permissions from macOS. You can do this from the initial first run
screen when Umbra launches for the first time.
If you later revoke the permissions, or choose the wrong option in
the system dialog initially, Umbra won't be able to request the
permissions again.
To resolve this you can run the following 2 commands in your
Terminal, which will reset Umbra. The next time you launch Umbra
you'll be able to request permissions from the first run screen
once again.
tccutil reset AppleEvents com.replay.Umbra
defaults delete com.replay.Umbra