What is Harper Lee's message/theme on race?

In the novel, Lee shows that racism is born of prejudice, intolerance, and ignorance. There are elements of racism in Maycomb that are quite obvious. The white and black populations live in separate areas. This has to do with economics as much as race. In addition, there are elements of racism that are unsaid and simply understood. When Tom is charged with rape, everyone (black and white) knows that he will have a hard time...

In the novel, Lee shows that racism is born of prejudice, intolerance, and ignorance. There are elements of racism in Maycomb that are quite obvious. The white and black populations live in separate areas. This has to do with economics as much as race. In addition, there are elements of racism that are unsaid and simply understood. When Tom is charged with rape, everyone (black and white) knows that he will have a hard time getting a fair trial from an all-white jury. That is, even the members of the jury silently acknowledge their own prejudices. Atticus presents a logical and strategic case but the jury can not, or will not, overcome their prejudices. In Chapter 23, Atticus notes, "The one place where a man ought to get a square deal is in a courtroom, be he any color of the rainbow, but people have a way of carrying their resentments right into a jury box." 

Some members of Maycomb will admit their prejudices. Some will not admit a thing, even though they perpetuate racist thinking. And, there are some hypocrites who claim that they have no racist thoughts but they clearly still hold some prejudices. Consider Mrs. Merriweather's hypocrisy in supporting the Mrunas but ignoring the needs of the black community in her own town. 

In Chapter 23 (same section), Atticus adds, " As you grow older, you’ll see white men cheat black men every day of your life, but let me tell you something and don’t you forget it— whenever a white man does that to a black man, no matter who he is, how rich he is, or how fine a family he comes from, that white man is trash.” Atticus is saying that, because of racism and intolerance, black men (and women) are at a disadvantage. They do not get the same rights and opportunities and understanding that whites in this society would get. So, when a white citizen takes advantage of this, Atticus says this is trashy. In this sense, racism is more than just having evil thoughts. Racism manifests in real ways and affects real lives. Tom is accused and killed because of racism.