If you wanted to strike a note of black humor, a theme song could be Europe's "The Final Countdown." I think that it would be more of a sardonic inclusion, but given how there seems to be this friendly bond between soldiers, the reality is that their interactions of cordiality and card playing are on a "final countdown" of sorts that ends up being non- negotiable.
On a more serious note, I would suggest that Pink Floyd's "Comfortably Numb" would be an appropriate choice of theme song for the short story. The ending where Bonaparte looks up at the stars and recognizes his own smallness is a representation of the numb feeling he holds towards his role in the war and his participation in what is essentially state- sanctioned murder. Bonaparte is reduced to nothing more than a functionary who has to "give him another." At the end of the story when Bonaparte suggests that ‘‘anything that happened to me afterward, I never felt the same about again," it becomes clear that he is numb as a result of what he did. This would be where Pink Floyd's song about alienation and what it means to be a distant part of the world around an individual is applicable. The lyrics and the general mood emphasized by the mood of the song and the guitar solo at its end can capture the reality of what soldiers like Bonaparte and Noble felt. This emotional experience helps to make "Comfortably Numb" an appropriate theme song for O'Connor's short story.