This question has been asked and answered time and time again on eNotes. Here is a link for you: http://www.enotes.com/to-kill-a-mockingbird/q-and-a/what-major-themes-work-how-each-themes-portrayed-22369
While there are many themes in almost every novel, there is usually one dominate one. In To Kill A Mockingbird, it would be lessons learned growing up. This broad theme includes many of the important life lessons Scout and Jem learn as they mature. For example, Jem learns that most of the people of Maycomb do not share his father's opinions regarding race. Before his family is threatened, Jem believes that his neighbors are "good" people who will support Atticus and find Tom innocent; however, most of the Finch family's neighbors are angry when it becomes apparent that Atticus intends to actively defend Tom Robinson. It is only with Miss Maudie's help that Jem comes to understand that there are "good" people in Maycomb, like Judge Taylor who appoints Atticus to the case because he knows Atticus will work on Tom's behalf. Scout comes to understand one of her father's most important life lessons: you never really understand a person until you walk around in his/her skin. Scout learns to understand the actions of difficult people like her teacher Miss Caroline, Aunt Alexandra, the Cunningham family, and even the malevolent Mrs. Henry Lafayette Dubose. Furthermore, when Scout stands on the Radley porch and views Maycomb from Boo's point of view, she better understands him and why he chooses to remain in the safe isolation of his home. Most importantly, the children come to understand the significance of Atticus' warning that it is a sin to kill a mockingbird. When the children receive rifles for Christmas, Atticus tells them they can shoot at other birds, but not mockingbirds, for they do no harm; they simply make beautiful music. That basic description can be applied to both Tom Robinson and Boo Radley. Tom helps Mayella Ewell because he feels sorry for her, and that act of kindness leads to his death. Boo Radley risks his life to save Scout and Jem from Bob Ewell's vicious attack. Sheriff Heck Tate says that exposing Boo's heroic deed will thrust him into the limelight, thus shattering his safe life of obscurity. Because they have matured and witnessed the ugliest side of human nature- hate, discrimination, envy, etc., they immediately understand the profound significance of this statement. In fact, Scout comments that there is not much else for them to learn. Thus, To Kill A Mockingbird is a novel about the lessons learned growing up.
good and evil / the power of negative racisim
More than one themes: -
1. Racism - againts Tom Robinson and other black people. Also black people were being racist again white people ( Lula).
2.ageism - against Scout, Jem, mrs. Dubose, etc.
3. sexism - against women ( women are not allowed in the court house)
4. Classism - ( ex: Walter Cuningham)
Major theme - that there are people in the world who represent mockingbirds ( TOM and BOO)