A theme (also known as a motif) is the main driving idea behind a poem. A theme or motif is not a summary of the poem, or a detail from the poem, but rather the emotion or motivation behind the poem. The theme might be "unrequited love" or "the power of traditions," but it wouldn't be "putting up walls between property lines," or anything else that specific.
The answer could be that the author wanted to show readers how unjust, cruel, and incorrect prejudices can be. After the main conflict is resolved, the narrator goes on to prove how incorrect everyone's prejudices about the Duvitches were.
I read that book last year :)
