In an abstract sense, the themes of this chapter are the themes of the entire book; they are just given a different emphasis. That said, I'd say that the themes shown in this chapter are first of all the pain caused by racism, the different forms of cowardly hypocrisy, and how heroism can take different forms. The racism is shown in the gossiping, as is the hypocrisy. Ewell's spitting at Atticus is definitely cowardly, and Atticus' calm response is a kind of moral heroism. Likewise, Judge Taylor's choice of Atticus as a defender is a kind of quiet heroism; he's setting his town up to change.