How do I give a boss mob its own boss theme?

I need to know: How do I make a boss mob have a boss theme in-game? I'm doing this for the remaster of my old Learning with Pibby: Apocalypse mod called "Pibby's Apocalypse" which has a working Wither Storm fight, with the Wither Storm corrupted by the Darkness, and unlike the original, it does actually work.