As well as the CSS wiki that @NoSi linked there is also the Share your CSS topic which has tons of other little tweaks and examples of things you can do.
You can access the development tools (Help > Toggle Development Tools) to find specific elements and classes.
There are two files in your joplin profile. userstyle.css is for the rendered markdown and userchrome.css is for the overall Joplin application including the markdown editor. Both can also be accessed in the way that @dpoulton already mentioned.