distribute custom.scss for quarto vignettes via R package

Hi Christophe ! @cderv

Thank you for following up on my question at https://github.com/quarto-dev/quarto-cli/discussions/2307#discussioncomment-13803117!

But you want this custom.scss to be in your package as an extension you would install?

I did create a custom format with the _extensions but it does not work for R package vignettes IIUC.

Or ship within the package ? Is this custom.scss used only for the vignette ? Or is this something you want to make available to others, and why you have a custom format ?

Yes, I do have an R / Bioconductor package that would distribute the custom.scss within it. The custom.scss would be used for both the package vignette and made available for others to use (though the latter is higher priority).

The issue does not name any files, tests, or entry points. First clarify how quarto-r packages currently distribute R package vignettes and whether the custom.scss must also be available as a reusable extension; done means documenting or implementing a supported distribution path for both stated uses.