Tools > Options > Appearance > Custom stylesheet for Joplin-wide app styles (Edit) & paste it in. Save the css file and RESTART (File > Quit) Joplin.
It's not quite as simple as that. This post gives a method of finding out what the css elements/classes are for the interface (also works for the viewer pane as well using userstyle.css). Also have a look at the "tips & tricks" area of the forum, there's lots of css stuff there.
I'm not so sure that would be such a good idea. Custom css has been possible with Joplin for ages but it has always been on the condition that no-one expects the css elements, classes, or even the actual interface to remain constant. It's unsupported and that prevents the devs being "hamstrung" by old code. If becomes a part the program there is then a need to support it. I would not be surprised if an interface allowing full and intuitive customisation of all elements would be larger than the current code for Joplin itself!!!
To be honest, "vanilla" Joplin is really rather good. It's "icing on the cake" to be allowed to abuse it so much with user css styling.