Hello Again!
I see what you mean about the event only having one category.
We are just echoing this for the class:
echo $wp_query->current_post;
On this template:
/events-calendar-pro/views/pro/widgets/mini-calendar/single-event.php
You can modify that template there to find all the categories of the event and put them into the class.
If you follow our themer’s guide:
https://theeventscalendar.com/support/documentation/events-calendar-themers-guide/
It will show you how to move that template mentioned above to your theme to safely edit.
Let me know if you have any follow up questions. However, there is not much more we can provide on a customization.
Thanks