What is the theme in Tunnel in the Sky by Robert A. Heinlein?

Survival is the main theme in the book. In order to get a job in the Outlands, a person in Rod's time has to take a survival course. Rod chooses to take the course in high school. The course teaches him what he should do to survive in an unfamiliar environment with unknown dangers. At the end of the course, Rod is expected to take a test during which he will be delivered to an unknown planet and left to survive for anywhere from four to ten days.