I am not using desktop, but kiosk mode.

I tried this thing: https://raspberrypi.stackexchange.com/q ... play#46229, however the directory /sys/class/backlight is empty.

However I found another way. Since I am using full screen web technologies, I am able to use opacity on a black background, this works especially well on OLED. This display looks really nice the night when strongly dimmed.