There is a thermostat.
Older units use a sensor tube that runs from the inside of the
fridge to the gas valve. When the inside temperature is cold enough, only a pilot flame burns. When the inside temperature rises, the thermostat opens the
propane valve and a larger flame heats the ammonia gas
generator to start the
refrigerator process.
I guess in cold weather you could turn the main
propane valve off and save the amount of
propane it takes to run the pilot
light. But what would be the consequence if you forgot to get it going when the weather warmed?
For newer units, many have electronic controls that eliminate the pilot
light. I can't see where there would be any saving with such a unit to turn the gas off during cold weather. However, I have never owned this type of
refrigerator.