In the beginning of the story we are introduced to the theme of the book. Which describes racial prejudice and the hardships of a colored person. In the time period of the book, the 1930's, is when colored people were fully removed from society. Resulting in segregation.
The hero is born not made. Atticus is that hero, standing up for a colored person in need. Disregarding all standers of segregation. He knows what will come of his decision but still continues to defend an innocent man.
The Great Depression was also a struggle to face for the characters. Will lose of money and stark markets crashing the worst was yet to come for many. Describing the authors tone of the book through the problems each character faces. Thus the authors tone is serious.