There were a few discussions a few months ago on how to improve theming in general but the conversation seems to have stalled on that front recently (although I seem to recall there were some changes to the Joplin theming or background CSS framework in some recent commits)
It mostly surrounded making it easier to target certain areas of the application with CSS as well potentially setting things up for a simple "theme designer".
Have a read of these if you are interested:
- https://discourse.joplinapp.org/t/ideas-about-joplin-theming/194
- Plugin CSS Improvement Ideas
- Talking about Markdown's CodeMirror theme update - #2 by CalebJohn
As for "installing with a click" - there are already a few themes created as Joplin plugins that you can find and enable.