The key combination to enable/disable keyboard backlighting varies depending upon product line and even specific model. Look at this Dell document.  Try F10 or Fn + F10. Hold F2 key during bootup to enter BIOS to see if it can be enabled/disabled there.
When I turn off the computer, the keyboard briefly lights up. How do I turn on and off the lighted keyboard?
Dell 5290 2 in 1 lighted keyboard