What is the music being played when Tatara learns to dance from Kiyoharu?
I don't know the answer to your question but you can't find it because Yuuki Ayashi, the composer of the background music, hasn't released an OST yet.

I was searching for background music too... ^^ Not that particular one, but several pieces of music in the score of this anime.

Guess we have to wait a little more before finding it.