The strongest theme in the story is the power of interpretation. Human beings can receive infinite amounts of input, but if anything directly effects ourselves, we twist and contort this information to reach the conclusions we desire. It woudl seem it is impossible for us to be objective in matters concerning ourself.
The mind sees what it wants to. Of all the sensory input he contorts throughout the story it first starts with the scout. He believed that he was encountering a chance to prove his worth and dedication to the cause. Instead he is duped by a spy seeking to expose partisans.
I would say the main theme is appearance vs. reality. In this story the theme develops in Farquhar's dream of escape that takes up the bulk of the story. The details of the story make it seem like it is really happening, but in the end you discover that it is merely a dream that passed through the man's mind in a matter of seconds. It causes the reader to wonder if the protagonist recognized it was only a dream.