Auto dark mode extension

[mvanrongen]

I've added an auto dark mode extension to my in-progress coding for research materials.

Works quite well out-of-the-box. Two changes in _quarto.yml, an added filter and an added theme:

filters:
  - courseformat
  - auto-dark

format:
  courseformat-html:
    theme: flatly

edit:

Needs a tweak in the extension itself, because otherwise the background colour of the sourceCode blocks is also black. This makes it hard to see the delineation of the code chunks. In the auto-dark-mode.css file, add:

  /* Re-enable source code block backgrounds. */
  div.sourceCode {
    filter: invert(10%);
  }