Is this story mainly about survival or the power of nature?

Jack London actually wrote two versions of "To Build a Fire," and it is arguable that the prominent theme depends on which version one is considering.
The original story from 1902 is the lesser known of the two. In this first draft, the protagonist has a name, Tom Vincent, and he is not accompanied by a dog on his trek. The weather is cold, but not as extreme as in the later draft. Tom Vincent is able to successfully build a fire to save himself though he does fall victim to frostbite. Though this story does allude to the power of nature, it is more thematically concerned with Tom Vincent's survival and ultimate consequent respect for nature.
In the second, 1908 draft, the protagonist has no name, and he is accompanied by a dog who relies on its instinct to survive. The protagonist is arrogant and dismisses the advice offered by an old-timer who understands the danger of what the protagonist intends to undertake: a solo day's travel in subzero weather through barren countryside. After failing to build a fire to save himself after he is partially immersed in water, the man freezes to death. This version of London's story is more concerned with the power of nature and the shortsightedness of those who underestimate it.