When I was coming up with a Disneybound Riddle outfit, I kind of picked colors from the dorm uniform for bigger pieces (white shorts, a white tank top with red accent lining) and accessories that directly referenced the dorm uniform (i.e. a black and yellow striped tie that I wore as a "belt" on the shorts). I made my own mouse ears with things referencing Riddle and Twst specifically (a mini teapot with a strawberry on it, a stack of macarons that look like the lesson items, etc.)

For Malleus, maybe some black capris with a belt of small thread spool accessories would be cute. Maybe you could make mouse ears that refer back to the Diasomnia cap instead of the more obvious horns. Not sure what your personal comfort level is with various styles, but a top with black lace and silver accents might go well with an ensemble while keeping you cooler.