I'm having trouble adjusting the backlight on my LCD screen in RC 1 with
XFCE4. No matter what I've tried so far, it stays where it is. With
Ubuntu 16.04, things are pre-configured and it is just a matter of
pressing the right function buttons. However, with Devuan Jessie RC 1
the right commands are hard to figure out.

Here is what I've tried so far to change the backlight intensity. None
have any effect:

$ xrandr --output default --brightness 0
xrandr: Gamma size is 0.

$ ls /sys/class/backlight/

$ xgamma -gamma 0.9
-> Red 1.000, Green 1.000, Blue 1.000
<- Red 0.900, Green 0.900, Blue 0.900

$ xbacklight
No outputs have backlight property

I've also tried adding "acpi_backlight=vendor" to the kernel boot options.

What should I be looking at or trying so as to be able to increase or
decrease the backlight?

Regards,
Lars

PS. The RC1 installer is much improved for this notebook. It found
the wireless, for example