Show the configured brightness
cat /sys/devices/platform/dell-laptop/leds/dell\:\:kbd_backlight/brightness 0
See the maximum brightness
cat /sys/devices/platform/dell-laptop/leds/dell\:\:kbd_backlight/max_brightness 4
Set the desired level of brightness
$ echo 4 | sudo tee /sys/devices/platform/dell-laptop/leds/dell\:\:kbd_backlight/brightness
References
https://askubuntu.com/questions/1076393/dell-keyboard-backlight-is-not-working
https://www.unixtutorial.org/controlling-dell-laptop-keyboard-backlight-from-command-line/