PrawnOS-nonfree/filesystem/resources/InstallResources/xfce-config/brightness/backlight.rules

/etc/udev/rules.d/backlight.rules

ACTION=="add", SUBSYSTEM=="backlight", KERNEL=="backlight", RUN+="/bin/chgrp video /sys/class/backlight/%k/brightness"
ACTION=="add", SUBSYSTEM=="backlight", KERNEL=="backlight", RUN+="/bin/chmod g+w /sys/class/backlight/%k/brightness"