Myujth

Is there a way to change faces one at a time, outside of customize?

In my 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.

Any suggestions?

You can use custom-set-faces or custom-theme-set-faces, with a list of one face - no problem. Or you can use modify-face. Or, as @erikstokes mentions, you can use set-face-attribute.

Note that the Elisp manual, node Attribute Functions, says this about set-face-attribute (but it doesn't say why):

This function is mostly intended for internal usage.

Note too that in the case of user options (variables), there are the single-option functions customize-set-variable and customize-set-value, which are companions to the multiple-option function custom-set-variables that is written to your custom-file (or init file). They are also commands, letting you choose the option using completion.

But for faces there is no such single-face companion function (and no command, other than customize-face).

set-face-attribute is the function you want. For example to make comments green and bold:

(set-face-attribute 'font-lock-comment-face nil
 :foreground "Green"
 :weight 'bold)

There are also helper functions set-face-foreground and set-face-background for the common case of just changing the color of the face.