To me, one of the central themes of this unforgettable story of man against nature is the way that this story reflects the Naturalism of Jack London. Naturalism is a philosophical idea that presents human beings as subject to natural forces beyond their control. This idea is clearly one that is central to "To Build a Fire," as the central protagonist is very over-confident and blasé in his approach to Nature - an over-confidence that ends up costing him his life. Note how the protagonist is presented as being "without imagination":
He was a newcomer in the land, a cheechako, and this was his first winter. The trouble with him was that he was without imagination. He was quick and alert in the things of life, but only in the things, and not in the significances. Fifty degrees below zero meant eight-odd degrees of frost. Such fact impressed him as being cold and uncomfortable, and that was all. It did not lead him to meditate upon his frailty as a creature of temperature, and upon man's frailty in general, able to only live within certain narrow limits of heat and cold, and from there on it did not lead him to the conjectural field of immortality and man's place in the universe.
This quote, it seems to me, lies at the heart of the theme London is trying to establish: success in surviving the extremes of Nature, London seems to be saying, necessarily involves some kind of respect for the raw, elemental power of Nature and also man's place within Nature. Lack of recognition of man's "frailty" and "man's place in the universe" leads to foolhardy arrogance which puts our lives at risk. We underestimate the power of Nature and we overestimate our own position in the natural order of things, London suggests, at our peril.