I'm running Zorin OS 18 Core on a MacBook Air 11 mid-2011 and the keyboard back light won't turn on. I tried lightum, pommed, light, nothing worked, even with the Zorin OS backlight widget on. I don't think there is really a way to test the light sensor or the keyboard LED back lighting from a hardware perspective but what I can tell is that the caps lock LED works and nothing wrong showed in Apple diagnostic tool. I don't know what else I could try. Any idea?
Can you try:
sudo modprobe applesmc
If that loads the module and enables the keybaord backlight:
echo applesmc | sudo tee -a /etc/modules
There is also pommed in the repository, but I do not know much about it.