I want my CSS file to be shared by several Quarto R presentations, so this CSS file should be placed in a top directory. But Quarto seems to allow CSS files only in the same directory, or in a child directory. I have tried all these configurations without success, starting with the css option in the YAML header:
---
title: "my title"
author: "my name"
format:
  revealjs: 
    css: ...
---
- ../styles.css
 - "../styles.css"
 - ..\styles.css
 - "..\styles.css"
 - symlink 'style.css'
 - https://raw.githubusercontent.com/zoometh/thomashuet/master/teachings/stats/styles.css
 - "https://raw.githubusercontent.com/zoometh/thomashuet/master/teachings/stats/styles.css"
 - https://github.com/zoometh/thomashuet/blob/main/teachings/stats/styles.css
 - C:/styles.css
 - "C:/styles.css"
 
But also outside the YAML, in the narrative parts and code chunks
<link href="../styles.css" rel="stylesheet" type="text/css">
<link rel="stylesheet" type="text/css" href="C:/Rprojects/thomashuet/teachings/stats/styles.css">
<link href="https://raw.githubusercontent.com/xxx/xxx/master/teachings/stats/styles.css" rel="stylesheet">
``{css}
<link href="https://raw.githubusercontent.com/zoometh/thomashuet/master/teachings/stats/styles.css" rel="stylesheet" type="text/css">  
``
<link rel="stylesheet" type="text/css" href="C:/Rprojects/thomashuet/teachings/stats/styles.css">
So relative paths, absolute paths (local or GitHub), symbolic links, calls outside the YAML header, nothing works.