Dell keyboard backlight is not working

You may want to check the brightness settings for the Dell keyboard. The following procedure worked for me:

1. Show the configured brightness:

   $ cat /sys/devices/platform/dell-laptop/leds/dell\:\:kbd_backlight/brightness
   0
   

2. See the maximum brightness:

   $ cat /sys/devices/platform/dell-laptop/leds/dell\:\:kbd_backlight/max_brightness
   4
   

3. Set the desired level of brightness (here: 4):

   $ echo 4 | sudo tee /sys/devices/platform/dell-laptop/leds/dell\:\:kbd_backlight/brightness
   

Slightly related: You might also want to configure the time-out interval for the backlight, using the stop_timeout setting. See also the article How to Configure the Keyboard Backlight Time-Out Interval in Ubuntu Linux in the Dell knowledge base.