Nice research, @bizzaro. For some moments I thought you had found the secret door, the universal gateway to CSS files. I guess the experiments below took place on Linux:
In the first line you navigate 8 levels up, bringing you to /Users/<user>/, and then drill 3 levels down to the location of the CSS file. In Windows I have to go just 6 levels up (from somewhere below C:\Users\<user>\+apps\Joplin\?) to arrive in C:\Users\<user>\ before drilling down 3 levels. It's worse in macOS: 7 levels up (from somewhere below /Applications/Joplin.app/Contents/?) to the root directory, and from there 5 levels down to /Users/<user>/.config/joplin-desktop/resources/. I have once read a post that illustrates how to find out which directory Joplin is running from, but I couldn't find that back, regrettably.
Your second line does not work on any of my platforms, but I guess you didn't pretend it would work from Joplin anyway. I suppose this line becomes operational when the note is exported (provided that the CSS file is copied to the same directory). Correct me if I have misinterpreted you.
Anyway, thanks for your contribution.