Great! (My apologies for editing your path in the quote above a bit.) My paths are:

  • Win 10: C:/Users/<user>/+apps/Joplin/resources/app.asar/gui/note-viewer/
    ⇒ 6 levels upwards to the <user> folder
  • macOS Mojave: /Applications/Joplin-app/Contents/Resources/app.asar/gui/note-viewer/
    ⇒ 7 levels upwards to the root folder
  • Linux: No idea, but I expect Linux users can easily find out now via the development tools.
  • iOS: The CSS file is in the parent folder indeed, wherever that may be
  • Android: Something to be tried out by an Android user.

It works, but I'm pessimistic about the usability, though. I haven't found a way to update such a CSS file and get it synchronized. All my (rather dangerous) experiments failed.