Completing a character's personal "Reward Pass" will give you a unique theme for the game, (trying out designs at the moment, for UI and icons etc). 

Another way to obtain themes will be in the Affection Pass menu, there you'll be able to obtain bundles with skins and themes. 

Still looking into if the reward themes should be based on the Skin or the Character itself, (like Kelly having a playing cards theme & Jejune having a science theme) 

I'll probably end up making each theme based on their "OG designs".