What is the main theme in "The Snake" by John Steinback?

John Steinbeck denied that "The Snake" has a meaning, which also pretty well excludes having a theme:

John Steinbeck contends that he has told this story exactly as it actually happened in his friend Ed Ricketts's Cannery Row laboratory and denies knowing what—if anything—it means.

However, critics like to analyze the story from the point of view of Jungian psychological literary theory, so according to this approach, the story does have a theme. Briefly, in Jungian theory, the woman represents the Jungian archetype of the collective unconscious universal mind. Snakes are associated in Jungian theory with moments of sudden insight into this unconscious mind.

The snake-woman, who is described as lean with black hair close to her head and lithe movements, seems to have symbolically emerged from the primordial tidal pool that teems with life and when she departs, her almost soundless steps suggest a symbolic return to the same pool. Thus is she connected first with the universal unconscious mind and second with amoral, elemental primordial forces--those of the primordial pool.

The snake-woman and Dr. Phillips are sharply philosophically  opposed to each other. While she wants the snake and rat just for the self-directed pleasure of watching the "hunt," he is repulsed by harming creatures for selfish nonscientific reasons, although he accepts ending animal life for valid scientific purposes. This sharpens her symbolic representation of primordial forces and heightens the contrast between moral choice and amoral enjoyment, which dramatizes a thematic conflict between instinct and moral choice since, in Jungian constructs, snakes are emblematic of human instincts.

Putting all this together, it may be said that the theme is amoral primordial instinct versus higher intellectual moral reasoning. It is interesting to note, in light of this theme, that the snake-woman, representing amoral primordial instinct, never comes back and never compensates Dr. Phillips for the rats and the boarding although she promises she will. In addititon, he thinks he sees her from time to time, but it is never she. This circumstance enforces the supremacy of higher intellectual moral reasoning over amoral primordial instinct.