basicly, you can find your driver video on /sys/class/backlight
and just give value for executables path to running persen of light on your screen
[root@pingu ok]# echo > 90 /sys/class/backlight/intel_backlight/brightness
in case I use xbacklight and this error still show
[root@pingu ok]# xbacklight
No outputs have backlight property
So, the elegant solution is using acpilight tool, you can find it on AUR. Acpilight script will be fixed this shit.
just install and try your xbacklight now. Hope this can help