In Langston Hughes' poem, "Theme for English B," I believe the theme deals primarily with the perceived difference between the white and black experience...and that it shouldn't be "a black and white" issue.
To fulfill an English writing assignment, the speaker goes home and describes himself, providing the instructor with "truth." He speaks of his connection to Harlem, describes the things that he likes. He acknowledges that what is true for him may not be true for his professor, and perhaps it is because of race. As the only black student in his class, the speaker wonders if this paper that he writes will be "colored."
Looking deeper, the speaker realizes that once he turns the paper in, it becomes a part of his professor. What they do have in common is that they are American—in this way they are a part of each other, whether they like it or not: and he admits there are times when neither one of them likes that fact. However, while there are divisions in the American experience, the speaker believes they can still learn from each other—learning perhaps that what binds them is a truth for both of them...though not necessarily exactly the same. For instance, they can both like the same food or a Christmas pipe...some of the same things, even though they do not share the same race.
I believe that Langston Hughes recognizes the difference of race, while he wishes there was not an issue of color. But he is also realistic. There are other things—things that he and the professor don't have in common: the professor is white; he's older; and, perhaps most importantly, "somewhat more free," a glaring "truth" in 1951.
The theme is about trying to overcome the obstacles that separation by race creates.