What is the theme in Cry of the Kalahari by Mark James Owens?

The rugged life in the Kalahari is the main theme of the book. On their first venture into the Kalahari from the last vestiges of civilization, Mark and Delia take off on their third-hand Land Rover. Mark alights from the truck and walks across shallow water a mile wide, stomping the ground below looking for firm land for the truck to pass over. Still, the Rover would sink at times to its undercarriage.