i3wm, adjust brightness through command line
You may modify the value in /sys/class/backlight/{may vary}_backlight/brightness
example:
sudo nano /sys/class/backlight/intel_backlight/brightness
This tool does that https://github.com/multiplexd/brightlight then use it like this:
bindsym XF86MonBrightnessUp exec --no-startup-id brightlight -i 20
bindsym XF86MonBrightnessDown exec --no-startup-id brightlight -d 20