The most obvious theme of Jessie Redmon Fauset's Plum Bun concerns the racial prejudice that permeated every aspect of American society in the early twentieth century. Even those of us today who are educated on the subject can still find the depictions of bigotry in books such as this shocking, and this bigotry is even present in books of a slightly later period, like Ann Petry's The Street and Richard Wright's Native Son and The Outsider.

Fauset's novel, nevertheless, is realistic, and it accurately conveys the atmosphere of the US prior to the Civil Rights era.

But her themes are not limited to racial matters. Issues of social prejudice and gender are nearly as important in Plum Bun. In some sense, Angela, the protagonist, faces nearly as many problems due to economic concerns and the fact that she is a woman as she does due to race. Her wealthy boyfriend Roger will not marry her—not because she is African American (which he does not know or even suspect)—but because she is of an "inferior" social class. Because she is a woman, her happiness, in a time before women had achieved greater independence, is dependent more on the whims of men than it should be.

Ultimately the deeper theme is that severing one's links to family and background, as Angela does when she "passes," is wrong and self-defeating. This could apply to issues other than race, though the racial situation of the time exacerbates the problems that Angela incurs as a result of her "deception." Angela realizes that her sister Virginia is happier than she is despite Virginia's inability to hide the fact that she is African American and her lack of desire to do so. The issues and problems of Angela's life are only brought to a resolution when she accepts herself as she is—which is true of all of us no matter what the factors are that make one different from the "crowd."