Opening this to think about it further. See discussion 

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

<div type='discussions-op-text'>

<sup>Originally posted by **sagikazarmark** February  1, 2023</sup>
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.</div>

(More details shared later)

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

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

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

```markdown
---
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.
cderv
2023-02-02T17:28:53Z
cderv
enhancement revealjs
v1.4
open
[revealjs] Global custom theme configuration that is not override locally
https://github.com/quarto-dev/quarto-cli/issues/4193