Race and equality are themes in the play. Hally and Sam appear at first to be much closer than servant and master. In fact, Sam is much more of a father figure to young Hally than Hally's actual father. But there are moments in the play that highlight Sam's social inequality because of his race. One of the more subtle instances is discovered when Sam reveals why he could not stay and help Hally fly the kite. The park was a whites-only park. But the big racial inequality of the play occurs when Hally angrily demands that Sam now call him "Master Harold." Hally basically asserts his supposed superiority over Sam based on nothing more than race. It's a definitive moment in the play and forever destroys the relationship that they once had.
Another theme is father and son relationships. I touched on this in the previous paragraph. Hally's real father is a deadbeat dad. Hally knows it. It's why he asks his mom to come home from the hospital without his dad. It's why he's so angry after he finds out that his dad will actually be coming home that night. It's clear to the audience that Sam is much more of a father to Hally than anybody else. He listens to Hally, offers him advice, helps with his homework, and even lightly teases him when appropriate.