Howdy NPD,
No worries there, happens to all of us!
You can definitely add a class to all day events. It will require creating a theme override and thus some knowledge of PHP. Do you feel comfortable with those? If this is you first one I’d recommend first familiarizing yourself with Themer’s Guide . Once done there create an override for /the-events-calendar/src/views/month/single-event.php and navigate down to the third line from the bottom. This outputs the event classes for each event in Month View. Using the function tribe_event_is_all_day( $event_id ) you can tell if you an even is an all day one. If it is, output your special class.
Does that all make sense? Will that work for you? Please let me know.
Cheers!
– Brook