
We had a brood lamp so even if the surrounding room became chilly, it was always snug and warm in the box - and a heat lamp keeps the place dry too. I had a thermometer on the pig rail so I had a monitor in the box. I'd want it higher than 20C in the box. If puppies find it too warm, they will crawl under the pig rail - and I'd know so lower the heat/switch the brood lamp off (more likely during the daytime). We tended to avoid Winter litters. Also for mum's comfort while in with the puppies most of the time (early days), we'd drape a sheet across the box so she wasn't lying immediately under the lamp.
You'll know if the puppies are chilly as they will snuggle together, or close with mum. If they are too hot, they will scatter round the box. If just right, they will nurse and fall off, lying where they land off the teat

I'd rather err on the warmer side because once morbidly chilled, they won't nurse and could eventually die. There are different ideas about this, based on what works best for the individual litter/breed.