The theme of the poem "Harlem" by Langston Hughes is frustration.
In 1951–the year of the poem's publication–frustration characterized the mood of American blacks. The Civil War in the previous century had liberated them from slavery, and federal laws had granted them the right to vote, the right to own property, and so on. However, continuing prejudice against African Americans, as well as laws passed since the Civil War, relegated them to second-class citizenship.1
In the poem Hughes describes the deferred dream of equality. This poem was written almost 100 years after the Civil War and African Americans enduring segregation. I would use a reference to the deferred dream as the title of your essay such as postponed promises. Another idea is to take a more angry tone and title your essay something like Slaves to the White Man's World.