This is a script plus a systemd (user) unit to change color schemes when Night Color kicks in (to get dark mode).
This is basically a workaround until maybe someday Plasma gets proper support.
The color schemes are named in plasma-color-scheme-on-night-color.sh line 16 and 18. For a hack it's not worth makig it configurable.
Write the systemd config file in ~/.config/systemd/user/plasma-color-scheme-on-night-color.service and the script in ~/opt/scripts/plasma-color-scheme-on-night-color.sh.
Once the files are written, use systemctl --user enable --now plasma-color-scheme-on-night-color to turn on.