What is the theme or central idea of this poem?

That sonnet is "From the Dark Tower" by Countee Cullen, one of the leading black American poets of the Harlem Renaissance. The poem is about the experience of being black in a white racist society, being a member of a despised and oppressed minority, hoping for a more just future. Read it a few times with that in mind; you'll develop a better understanding of its central idea.