Well, considering the full title of this work of literature is Kon-Tiki: Across the Pacific on a Raft, we would be correct in assuming that this is quite a typical adventure story and the themes are as follows: survival and perseverance.
Why survival? Well, a Norwegian explorer named Thor Heyerdahl went across the Pacific Ocean on a raft made of balsa wood in 1947. He did this with five other men. The trip took over 100 days and the group went over eight thousand kilometers in that time. This was truly a test of survival in that the entire group needed to brave the elements, hunger, and each other for a long period of time. Even though the raft eventually crash landed at their destination, the group did survive. There is no doubt that survival is certainly a main theme here.
Why perseverance? Not only did the team of men persevere for 100 days on a raft out in the open sea, but also they persevered in their original, idealistic goal. The men (and especially Thor Heyerdahl) wanted to prove that South Americans could reach the Polynesian Islands by raft using only the tools available in the pre-Columbian era. Even though it was subsequently proven that South Americans of that time did not make the same conquest, Thor Heyerdahl certainly proved that South Americans certainly could have populated Polynesia. Not only did Thor Heyerdahl and his teammates survive, but they reached their destination.
In conclusion, it's interesting to note that both perseverance and survival (the main themes) were considered in titling both the book and the raft. Kon-Tiki is an old name for the Incan sun god who was worshiped in the Incan empire for many years. In a sense, this god survived and persevered just like the men on the raft.