Event Category as variable in twig template

I wish to have category-specific styling for each event displayed in agenda.twig, agenda-widget.twig and stream.twig.

I see these being added to the div class for each event in the original templates:

ai1ec-event-id-{{ event.post_id }}
ai1ec-event-instance-id-{{ event.instance_id }}

But I cannot seem to get the category ID of the event.

I’ve tried several different things around this general concept:

ai1ec-event-category-id-{{ event.category_id }}

What is the proper variable, and proper syntax, that would allow me to add the event’s category to the div so that I could style each category differently?

I can put {{ event.categories_html | raw }} into the loop item and return a link to the category – with the a tag having these classes added:

ai1ec-category ai1ec-term-id-3 p-category

What I’m trying to do is get ‘ai1ec-term-id-3’ added to the class on the outer div.

Thanks for any help.