David Kimura PRO said almost 4 years ago on FullCalendar with StimulusJS :
  Have a look at the CSS in the code at app/assets/stylesheets/application.css

The modal stuff isn't part of the Full Calendar ui, but some simple css can be used to create the modals. For a more complete solution, you should also have a cancel button which would then trigger for the modal to be closed.