The main theme in "To Build a Fire" is Man versus Nature, but in the sense of Literary Naturalism. The protagonist is not killed by nature, but simply is unequipped to survive in it; he does not take heed of warnings about the extreme cold and how quickly it kills, and so succumbs to it through his own failure. This shows the Naturalist and Realist tendencies of London's own writing; he viewed nature as a primal force, both unaware and uncaring about man's attempts to subvert or control it.
[The cold] did not lead him to meditate upon his frailty as a creature of temperature, and upon man's frailty in general, able only to 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.
(London, "To Build a Fire," jacklondons.net)
The protagonist has no imagination, but he is not preyed-upon by nature; he simply does not respect its power, believing himself as a thinking animal to be superior to simple natural forces. Instead, he is destroyed by an entirely passive state of existence: it is cold in the Yukon, colder than any human should experience, and he does not prepare himself for the reality of that cold. The state of cold is what kills him, but it would exist and continue regardless of his actions. Had he survived, the cold would still continue; it is only because of his mistakes that he dies, not because the cold holds any animosity towards him. This could change the overall theme from "man versus nature" to "man surviving nature's reality."