FWIW, I find it best to create a note with the Code prototype in my documents, and use that note for all css, whether for HTML or Markdown, etc. The internal css mentioned about can be copied there if desired. The css note (say “myCSS”) can be included in any template with
If needed, an export can export that note for external use in a set of exported notes.
This gives me more control than depending on some external referenced note, that doesn’t always work, as noted by the OP.
Edit: Or do what the post just below suggests. I use css notes in starting point files (aka stationary files) and when I don’t want to use the file used in the method below.