re: Is it possible to add a css class to a directory through the shortcode?
Sorry, no, not with the shortcode.
re: I realize this might need to be a snippet added in functions.php…
Maybe, depends on where you want to add the class. Maybe using a template override file will be simpler. Basically you copy the
card-single.php to one of the folders and then you can edit them as you wish… adding a class for example.
Hope that helps, let me know.