After fighting some hours with the brightness controls, I finally managed to adjust my backlight on Lxqt:
I installed light-git
light -U 20 decrease backlight 20%
light -A 20 increase 20%
light -S 50 set backlight to 50%
My original problem: trying to reduce backlight using fn keys only messes up with lxqt-brightness which does not affect the backlight at all, I only get a darker hard to see screen or it gets too much brighter.
Searching through Lxqt github, it appears to be a design problem?
Anyway my fn keys works fine on any DE that I tried, only Lxqt gave me this problem.
Btw, I have a Acer laptop with Intel sandy bridge gpu, nothing fancy really.
Can you laptop users chime in? I'm curious if it's just me the lucky one