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You can define a class to style your events in any way you want. If you don't assign a custom class or if the expression returns empty, the default class will be added to the event wrapper: event-type--default. When an event is creating (when drag creating for instance) class event-type--creating-new is added.

.event-type--my-custom-event.b-cal-event-wrap {
    // wrapper styles (only style when really needed)
    > .b-cal-event {
      // contains background color, etc.
      > .b-cal-event-body {
        // wrapper for your event content
      }
    }
}

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