[Revealjs] Allow customisation of callouts similar as in HTML

[cderv]

From discussion in #4755

• html format support some variables for callout (https://quarto.org/docs/output-formats/html-themes.html#callouts)
• This is because it is part of bootstrap SCSS file that we compile (
• For revealjs, this is just a <style> node that we insert in header of document (https://github.com/quarto-dev/quarto-cli/blob/main/src/resources/formats/html/styles-callout.html)

We should make that part of the default theme for revealjs, so that it is compile as a SASS theme, and variables can also be used (see #4755 (comment) by @EllaKaye)

[EllaKaye]

This would be excellent, thank you!

[sasja-san]

The callout blocks in my current project (right side) looks kinda bad. Will this change make it so they adopt the color/style of the "Card" elements seen to the left?

If not - is there some way to make a custom div which acts like a .callout-X but which renders to a Card?

[mcanouil]

@sasja-san what does your comment have to do with Reveal.js? I suggest you open a discussion with reproducible code.