Thanks, for this link, but the css includes fonts that are downloaded every time it is accessed. It should not include external fonts for 2 reasons: 1) what if you were to use it when offline, 2) and you might have issues using your own font (set in settings) with Joplin.
The point is to have it added to the app’s theming options, not having to mess with CSS files. Many users, including myself, have no idea how to use them. I would be willing to look into how to use them, but most ‘ordinary’ users won’t bother.
Furthermore, having to mess with CSS files is not exactly user friendly if you want to switch between themes depending on the time of day, or your environment.
Returning to Joplin after months of not using it due to it having become nearly unusable on Android due to the unholy combination of auto save and lack of undo function, I was pleasantly surprised that multiple themes have been added in addition to the much desired Solarized Light and Solarized Dark! And not just to the desktop client, but also the Android client (I assume iOS got it too?).