This book has multiple themes throughout the text, so picking the central theme is going to be up to individual readers. I do think that the themes of race and prejudice should be among the possible top choices. Racial relations and the strain that they can cause effect just about every interaction and conversation that the characters have in A Lesson Before Dying. Take the following quote as an example:
"I have no idea." He stared at me, and I realized that I had not answered him in the proper manner. "Sir," I added.
Grant is forced to add "Sir" for no other reason than the fact that the other guy is white.
Based on the title alone, I also think that a central theme is a theme about death and mortality. The book begins with a murder and ends with an execution; however, the book itself focuses on what it means to die with dignity and honor as a human being.
"Do I know how a man who is supposed to die? [. . .] Am I supposed to tell someone how to die who has never lived?"