[revealjs] Global custom theme configuration that is not override locally

Opening this to think about it further. See discussion

Discussed in https://github.com/quarto-dev/quarto-cli/discussions/4182

^{Originally posted by sagikazarmark February 1, 2023}
I want to include a certain piece of CSS in all of my slides (regardless of the theme), but still want to be able to use custom CSS on a per slide basis if I want to.

Currently if I set a default value for theme with a custom css file, it gets overridden as soon as I pick another theme in a slide (which is kind of expected).

I'd like to be able to include a CSS file in addition to the theme.

A good enough workaround for now is using include-in-header, but that can still be overridden.

(More details shared later)

Here is what I have in my _quarto.yml file:

format:
  revealjs: 
    theme: [default, custom.scss]

Then in any slide, if I want to use another theme:

---
format:
  revealjs: 
    theme: sky
---

I want to be able to include my custom.scss in all of my slides without having to remember to include it when I don't use the default value.

What I'm currently doing is I have include-in-header defined in my _quarto.yml to include custom CSS. I don't use include-in-header at the moment anywhere in my slides which is why I can rely on the default.