Pick your traits folder. Subfolders become slots, files become variants.
Use a number prefix on folders to control layer order (low = bottom):

traits/
  1-base/ naked.png  (always on)
  2-clothes/ hoodie.png, tape.png, robe.png …
  3-eyes/ yellow eyes.png, melting eyes.png …
  4-eye-items/ arrow.png, bandage.png …
  5-hats/ cap.png, halo.png, antena.png …

Folder named base (any prefix) is locked always-visible.
Background colors are built in until you export bg traits.

To skip this step, run python build-manifest.py next to this file — the page will then load traits/ automatically.