With local you mean project-specific?

for custom themes I just add the melpa package to my dotspacemacs-additional-packages and then added it to my dotspacemacs-themes

@pablore I tried i) adding a separate layer with this config

(defconst firebelly-packages
  '((firebelly :location local)))

And ii) an entry in .spacemacs.d/init.el.

dotspacemacs-additional-packages
   '((firebelly :location (recipe :fetcher github :repo "startling/firebelly")))

… both approaches failed.