Disable ugly background hook not working

According to the instructions...

> Or disable the background color completely in your .emacs.d/init.el:
>
> (add-hook 'mmm-mode-hook
> (lambda ()
> (set-face-background 'mmm-default-submode-face nil)))

However, this does not work for me. I'm using the `emacs` package provided by `Fedora 32`, and it happens on a "normal" `emacs`, and also in `spacemacs`.

```
$ emacs --version
GNU Emacs 27.1
[snip]
```
```
$ grep -A3 "default background" ~/.emacs.d/init.el
;; disable ugly default background on vue-mode
(add-hook 'mmm-mode-hook
(lambda ()
(set-face-background 'mmm-default-submode-face nil)))
```


Reproduce the issue with GNU Emacs 27.1 using the ~/.emacs.d/init.el snippet and mmm-mode-hook configuration shown. Start by checking how vue-mode and mmm-mode establish mmm-default-submode-face, then confirm that the background is disabled in normal Emacs and Spacemacs or identify the required configuration change.

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