Final answer:
The main theme of the poem is that racial prejudice is disgraceful and will eventually give way to equality, with the line "They'll see how beautiful I am/ And be ashamed-/ I, too, am American." best supporting this theme.
Step-by-step explanation:
The poem discussed likely touches on themes related to racial prejudice and the struggle for equality. Part A of the question asks to identify the main theme of the poem, and given the details provided about Langston Hughes's poetry, as well as the sentiments expressed in the other excerpts, one strong theme that emerges is: Racial prejudice is disgraceful and will dissolve in favor of equality. This theme encapsulates both the injustice of racial prejudice and the hopeful vision of a future where equality prevails.
For Part B, the detail from the poem that best supports this answer is "They'll see how beautiful I am/ And be ashamed-/ I, too, am American." This quote reflects the belief that recognition of the common humanity and American identity of black people will lead to a recognition of the ugliness of prejudice, ultimately resulting in a society that is ashamed of its past prejudices and embraces equality.