Is there a door at the top of the steps? Maybe a contact sensor would do the trick?
Another thought, I know it's not really home automation anymore at this point, and less fun, but what about a battery powered, motion sensing light?
Something along these lines:
https://www.amazon.com/eufy-Stick-Anywhere-Bedroom-Bathroom-Efficient/dp/B071CFBXXZ/ref=sr_1_8?crid=16N56A866EMHB
Or this:
https://www.amazon.com/Rechargeable-Motion-Sensor-Light-Sensored/dp/B0B5DNHFWL
