@R_HIDIA Also, you could try filling empty space on the tent floor with jugs of water. Will help retain what heat you do add to the tent. Not sure if just a light will be enough, because ironically as wasteful as those bulbs are, they are still not quite as efficient as a heater.
How about a small oil-filled heater? Those are pretty safe, if not power hungry.
Edit: Something like this would probably be ideal for a small to mid-sized tent.
Amazon product ASIN B009L40S0S
In case the amazon link doesn't show, search for NewAir AH-400 Portable Space Heater
Edit2: The reviews are a bit dissuading. Maybe not that specific unit, but something like it would probably do the job.