There isn’t a full list anywhere, but if you toggle development tools you can use the debug selector to click on any element and be given all the info about it (html element name, CSS class, etc.). Additionally you can change the CSS properties using the debug tools to preview how things will look. This is the best method for tweaking CSS with Joplin IMO.
The selector tool is in the very top left corner of the debug window and it looks like a mouse pointer clicking on a square.
Edit: more info