First, tents are pretty bad as insulation. Pretty much what you would expect from a thin piece of nylon. The physics of raising the temperature is that you have to gain more units of heat than your losing to raise the temperature.

A handy trick, if weight and bulk aren't too much of an issue, is to erect a tent inside a tent. Possibly both those inside another tent. The wall of a tent is pretty poor insulation but two or three tent walls with a space between them starts to add up as insulation. Siberians have been doing this tent within a tent thing for hundreds of years with hides. Hides are much more substantial than your typical tent material but then again they are fighting temperatures 40 below zero and fifty mile an hour winds. Three tents with one inside the other and a small fat lamp keeps temperatures in the forties.

Which brings up the other point; what is "warm"? If the outside is forty below keeping the inside a few degrees above freezing might be as good as it gets.

The principle of a shell within a shell is very useful if people are trapped at home in a blizzard. Shutting off most of the house, taking care to empty pipes so they don't freeze, and retreating to a small room can go a long way toward keeping warm. If you set up a tent in a small room you can gain even more.

Hint: the 'hook' portion of Velcro will grip most carpets and allow you to 'stake out' a tent and prevent it from wandering if bumped. A 4by4 piece of Velcro with a D-ring sewn in, prepared beforehand, will work on most commercial carpeting.

If you can isolate a small area and get good insulation between it and the outside temperatures your well on your way to being able to raise its temperature using a small heat source. A well built igloo, sometimes snow caves, might be raised to just above freezing by a candle or two even though the outside may be 20 below zero.