Automatically change Plasma theme between dark and light

Place timer and service file in the respective user directory $HOME/.config/systemd/user
Place the script into a directory and specify the path in the service file.
Change time in script if necessary (default is 9:30 p.m. and 6:00 a.m.)
Enable and start the timer:
$ systemctl --user enable themer.timer
$ systemctl --user start themer.timer