To the extent that historical events and eras have themes, the theme of the Harlem Renaissance was a sort of black nationalism and black pride.
The Harlem Renaissance was the time of the "new Negro." This was a time when African Americans were starting to feel that (at least in some places) it was safe for them to openly take pride in their race and their culture. This new pride (or this new feeling that it was safe to express that pride) showed itself in the fact that African and African American themes informed so much of the work of the artists and authors of the Harlem Renaissance. This was not a case where African Americans were copying white styles or trying to play to white audiences. It was a case when they were producing art that was by African Americans and for African Americans.