What is a major theme in "Hunters in the Snow?"
One major theme in the story is unintended consequences. The hunting trip becomes a disaster and because of the various factors, hidden truths come out, and a man might die. Each event is preceded by something that foreshadows it, but all the characters are so self-absorbed that they can't see past their own immediate gratification.
"You're a grown-up, Tub. You can take care of yourself. Anyway, if you think you're the only person with problems I can tell you that you're not."
"Is there something bothering you, Frank?"
Frank kicked at a branch poking out of the snow. "Never mind," he said.
(Wolff, "Hunters in the Snow," classicshorts.com)
Even though Kenny has a history of making cruel jokes, his sudden real action of killing the dog makes his joke on Tub seem real, and Tub shoots him. Because of the stress, Frank reveals his affair to Tub, and Tub reveals that he overeats on purpose. Kenny's probably death is a direct, unintended consequence of his constant cruelty to Tub, but he never considers that his jokes could have any real effect until it is too late.
Because this story is a true example of Existentialism in literature, I can appreciate the previous answers giving the detailed themes that they give, but I am going to be a lot more simple about it. The main theme of the story is hunting, just like we have to hunt for the meaning of the signposts covered in snow, we have to hunt for the senseless morality of the characters in this story.
There is a natural hierarchy here: one of the hunter and the hunted. In most cases between the three, Kenny is the hunter. Why? He is the most athletic, the most fit, the most agile, the most cunning, and the most intelligent. As a result, he "hunts" Tub: the least athletic, the least fit, the least agile, the least cunning, and the least intelligent. This is done through one practical joke after another.
In creating their own reality, "the hunters" create their own warped rules. One example of an absurdist, warped rule is that you can lead through bullying and keep close with "friends" through ridicule. No compassion is tolerated. The two other "friends" simply allow Kenny to die in the snow due to their warped rules. The bawdy jokes between the two living members are so coveted, that the need to help Kenny is ignored.
Bawdy bonding is worth more than life itself. Absurdism in its best form. What a bleak story! Further, the selfishness does not subside at the ending, as the characters flee from the North Star:
Right overhead was the Big Dipper, and behind, hanging between Kenny's toes in the direction of the hospital, was the North Star, pole star, help to sailors.
I think the theme of the whole story could be summed up in the following phrase:
pride and selfishness vs. friendship and compassion