The theme is best represented by Phillip's blindness. Phillip is a young boy who has been conditioned to believe that colored people are somehow beneath them because of their color and in his defense, much of the world, at the time, believed this as well, including Timothy. Timothy demonstrates "knowing his place" by always calling Phillip "young bahss" or "young boss".
Soon after the shipwreck and Phillip's head injury he becomes totally blind. After several months on the Cay Timothy teaches Phillip how to survive and be independent because he knows that his own death is near. Little by little Phillip begins to see Timothy as a friend and a mentor rather than a slave or a lesser being.
Phillip actually does not even see Timothy as "colored" any longer. He loses his feelings of prejudice and superiority as a result of being lost on this island.
Some other themes might be self reliance, friendship overcoming adversity, and grieving losses.