I am not aware of one and using a custom stylesheet is not a supported feature, so I cannot see there being any reason for any of the devs to collate a list of the css elements used.
The easiest way is to open the Development tools in Joplin (Help > Toggle development tools) and use the "picker" ([1] in picture) to select an item you are looking to customise. This will show a flyout ([2] in picture) and the code ([3] in picture) that can help you to determine the element / class you wish to modify.
