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

The rugged life in the Kalahari is the main theme. 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. After swatting masses of mosquitoes, they wash the crusted mud off their hands in faces in mud puddles. They sleep atop their truck not knowing what or who may approach at night. The terrain is so adverse that on some days, they would travel only a few miles. Their scant directions to the wildlife reserve include a left turn at a broken palm.