this is one issue that was hard to find a suitable answer for
my screen was stuck at maximum brightness (not so bad during the day, but at night was blinding)
i tried everything i could find on the interwebs,
and there was quite a few suggestions as this is a common issue, all to no avail
finally i found this solution:
open the terminal and input
xrandr | sed -n 's/ connected.*//p' | xargs -n1 -tri xrandr --output {} --brightness 0.6
where 0.6 at the end of the string = 60% of max brightness,
this can be changed to whatever you wish between 0-1 ie 0.8 = 80% of max, 0.4 = 40% of max, etc.
so i simply saved this line in libre office and input it in the evening once and am done,
next morning it boots to max for the day
i hope this helps someone out (it seems to be a catch-all and may ork for any set-up),
as it is a common struggle and can be frustrating
