Для ноутбуков я только что узнал из man xrandr:
--brightness brightness
Multiply the gamma values on the crtc currently attached to the
output to specified floating value. Useful for overly bright or
overly dim outputs. However, this is a software only modifica‐
tion, if your hardware has support to actually change the
brightness, you will probably prefer to use xbacklight.
Итак, я попробовал
xbacklight -get
xbacklight -set 70
, и он работает!