Courtesy of @meuh, I managed to get Ubuntu to automatically switch between the "light" Window Theme and the "dark" Window Theme (see over here)... But for some reason, systemd does not seem to be starting my timers after a reboot.
If I restart my system and run systemctl --user list-timers, it tells me that there are no timers... If follow this up with systemctl --user restart dark.timer light.timer and then run systemctl --user list-timers again, my timers show up correctly.
I have repeatedly run systemctl --user enable dark.timer light.timer before I restart my system, but as stated above, systemd seems to "forget" this and will not show these timers under after I have used the systemctl --user restart dark.timer light.timer command...
I feel confident that the solution is something trivial, but I have no idea what that solution might be, so any help the Community can provide would be appreciated.
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Output of ls ~/.config/systemd/user/*.target.wants:
ls: cannot access '/home/gregory/.config/systemd/user/*.target.wants': No such file or directory