Adventure and obstacles
The novel is set as a series of trials and tribulations as Stephen approaches his real adulthood. When his dad asks him to come help him in their village instead of going back to school, Stephen accepts the invitation to a new life, and he finds out that his new life has a new added feature that life didn't have in his original village: girls his own age. In other words, Stephen is a hero who goes on an adventure to identify more closely to his father, and he wins a host of new friends, some of whom are even girls his own age.
Disease and death
The thirst for adventure that defines Stephen throughout the book is counter-balanced by the inclusion of disease, plague, and death. When Stephen has Tuberculosis, he is isolated, which adds loneliness to his list of problems, which can be a very scary thing for a young person with a deadly illness. Then, after all that sacrificed time with his loving sister, she dies of Tuberculosis anyway. This horror and tragedy is in the novel because it's a real part of human experience.
Family duty and identity.
The plot detail that speaks most to this theme is that Stephen leaves his mother behind to go work with his father, which is a picture of him performing his sacred duty to become a man, instead of regressing into a different kind of life. As far as his identity is concerned, Stephen identifies as an painter, and he often spends his days on the beach, swimming alone, and then painting various beach-scapes. In other words, he finds his identity in his unique artistic expressions, but he still commits himself to his duty to do what is best for his family.
