Identify the structure of the poem "Song for a Dark Girl" and explain how the structure of this poem helps to accentuate its theme.

“Dixie” is a Southern anthem that celebrates life in the Confederate states. This popular song speaks about how fabulous life in the South is for those born there. It began in minstrel shows where performers dressed in blackface to emulate African Americans of the time. The last two lines of the Chorus in “Dixie” are as follows:

Away, away, away down South in Dixie,Away, away, away down South in Dixie.

"Song for a Dark Girl" by Langston Hughes begins each of the three stanzas with a reference to or echo of this popular Southern song: “Way Down South in Dixie.” This direct reference to “Dixie” sets up the rhythm of the poem and an allusion to “Dixie.”
Minstrel performers made it seem to audiences that African Americans really loved living in the South, though this was rarely true. The second line of each stanza, which are in parentheses, references physical or emotional pain. This belies the idea that Dixie, or the South, was a joyous place for African Americans. It was often a brutal place with little hope. The lynching that is this poem's central image is evidence of this.
The simple iambs, or two syllable feet, of this poem create a bouncy feel at odds with the dark subject matter. The structure of this poem closely mirrors that of many popular and upbeat songs of that time. This juxtaposition between a celebratory song-pattern and this poem of anguish sets up the idea that life in Dixie was wonderful for white people but, in contrast, full of pain and misery for African Americans.
See more about minstrel performers and blackface at the reference link provided.

The poem "Song for a Dark Girl" by Langston Hughes consists of 3 quatrains, or stanzas with four lines. The quatrains are written in irregular iambic trimeter, which means that there are three pairs of iambs—an unstressed syllable followed by a stressed syllable. To indicate this, I have bolder the stressed syllables in the following line:

Love is a naked shadow.

This line also includes an extra syllable, which is called a feminine ending. Hughes deliberately breaking the meter is what gives this poem its bluesy feel.
This song is a blues song where the speaker is mourning the death of her lover, who was lynched. The form of the poem, then, reflects the theme of the poem because of its blues rhythm.
The speaker expresses how the racist violence of "Dixie" breaks her heart and disfigures her love and soul. Hughes is trying to say that what is happening to African Americans in the South should break the hearts of people everywhere, just like the speaker of the poem.