The general idea is that the default styling should be enough for most people. The CSS tweaking is an extra "advanced feature" that allows modifications of stuff but comes at the risk that things may change without warning which is why you won't find any "official" documentation about how to change things.
There are a bunch of resources on the forum like the one I linked to (mostly posts you can find in the "wiki" heading at the top) where people share how to style various elements of the app. It does require some CSS knowledge though. For example Joplin Customization
Pretty much, yes, although you won't be able to just throw it in anywhere, you still need to inspect the elements manually if you want to style something specific.