I think the theme has a lot to do with appearance vs. reality. Peyton so desperately wants to be home with his wife and children that he focuses completely on that unrealistic expectation. In doing so, he is able to recreate his hopes in the last minutes of his life. Of course, his imagined escape and homecoming are full of clues that his experience is not real. The escape from the volley of shots fired at him, the idea that he can see the color of the eye of the sharpshooter and the insects on the trees are all clues that his imagination is more real than reality. The final blow comes when we realize the entire sequence of events in not real, they only appeared in Peyton's imagination.
This is a tough nut to crack. It would be easier to identify the subject of the story, that being the fragility of life and the subjective nature of our interpretation of it.
A theme by definition is a complete statement, and usually takes a stand in one way or another. To transcribe the above subject into a theme, one might formulate the same idea in this way:
'One of the basic qualities of life is its fragility, but this transient aspect is one of the most difficult ones for people to accept.'