Thanks! I created that file (.config/joplin-desktop/userchrome.css) and entered the css you provided. Then started Joplin and I didn't see a difference, not in the MD editor and not in the wysiwyg so I opened the settings, appearance, advanced- 'Custom stylesheet for Joplin-wide app styles' and the newly created css file opened so Joplin should be aware of it.
(it's a flatpak)