How can I use quarto to render a revealjs report with serif fonts using scss?

Question

I would like to have my quarto/revealjs slides use serif fonts. I know that I can specify the font using the mainfont option:

---
title: "blah"
format: 
  revealjs:
    mainfont: times
editor: source
---

or I can set the $font-family-sans-serif: option in a scss file to use a font with sarifs:

---
title: "blah"
format: 
  revealjs:
    theme: [default, custom.scss]
---

/*-- scss:defaults --*/

$presentation-font-size-root: 30px;
$font-family-sans-serif: 'Times', serif; 

/*-- scss:rules --*/

.red {
  color: red;
}

I would like to get all my styling into the scss file but setting the "$font-family-sans-serif:" to use a font with serifs seems broken.... How do I specify that a theme should use a serif font?

I know there is a serif theme but I can't figure out where it lives on my machine. Can somebody please tell me the names and locations (on a Mac) of the theme files that quarto uses for revealjs?