Is there a way to change faces one at a time, outside of customize?
custom-file I see a big blob of face customizations that starts with:
(custom-set-faces ;; custom-set-faces was added by Custom. ;; If you edit it by hand, you could mess it up, so be careful. ;; Your init file should contain only one such instance. ;; If there is more than one, they won't work right. ...
So I suppose
custom-set-faces is not the right function to use for changing just one face, but I'm not sure what is.