Certainly racism is major theme in Light in August. We see light-skinned Joe Christmas, who passes as white, become the prime suspect in the murder of Joanna Burden when Joe Brown tells the sheriff that Christmas is black. Brown, of course, is trying to save his own skin since his own alibi does not clear him. As he says, "Accuse the white man and the nigger go free." Once this is said, Christmas is guilty in the eyes of the white Jefferson community, and Brown, a white man, is no longer a suspect.
Racism plays a role in Christmas' upbringing as well. Christmas rebels accepting his white foster parents' food, religion, and name. When he finally defends himself against the violent McEachern, Bobbie, the prostitute Christmas had hoped to run off with and marry, becomes angry at Christmas for making a scene and begins to call him "nigger" and other racial slurs. She is angry because his attack on McEachern was done in a public place, and her illicit business operations might be exposed. In her anger, she plays the race card even though his earlier confession to her that he might be part black did not bother her at all.
This use of the race card against him becomes a pattern in Christmas's life, and each time it is used, it has devastating consequences for Christmas.
Faulkner shows us all kinds of racists: from Christmas's fanatical grandfather who allows his own daughter to die in childbirth because he believes she slept with a black man to Joanna Burden whose family believed that blacks were the white man's curse. I don't think he is advocating specific political changes. Faulkner firmly believed that the South would have to change on its own accord. Perhaps his writing was his way of provoking that change. It is clear that he wants us to understand the damage that racism does to our fellow man. Many of Joe Christmas's flaws can be attributed to the sicknesses in the society in which he lived.
As Hightower says, "Poor man, poor mankind." By negative example, Faulkner makes a strong argument for compassion, tolerance, and understanding.