What would you say is the main theme of this book

gradesaver has a list of detailed themes for the novel, simply follow the link below;

http://www.gradesaver.com/to-kill-a-mockingbird/study-guide/major-themes/

Of the eight detailed themes, I see Atticus' desire for his children to view the world from all perspectives (not just their own, or to rely on someone else's) as the main theme. It's a fantastic lesson.