what is the theme of the novel

There are many themes you can consider. Racism is a big theme in the book. Maycomb is an Alabama town that seems quite typical for a small southern town of that era. Maycomb is old and set in its ways. Like many small towns, there are few secrets and everybody seems like they have been there forever. Check out Scout's quote from the first chapter,

"Maycomb was an old town, but it was a tired old town when I first knew it... There was no hurry, for there was nowhere to go, nothing to buy and no money to buy it with, nothing to see outside the boundaries of Maycomb County..."

Being white meant a physical and emotional separation from the black population. Scout and Jem at least got to experience a housekeeper /mother-figure in Calpurnia. Years of ignorance, hatred, social conditioning and rawness from the Civil War were, and apparently still are, tough to change.