The tool uses the files in /sys/class/backlight/ provided by the Linux Kernel ACPI module. Currently, this only works with intel graphics, because that is what I have, hence I know the names of the files relevant to me: /sys/class/backlight/intel_backlight/{brightness,max_brightness}.
This can easily be adapted to other interfaces.
Eventualy, I hope to make this work for other graphics cards.