You might consider creating a custom page type based on the
CMS.BookingEvent to add fields that better suit your situation (or add fields to the existing page type). If you add an
EventCategory field, then you can grab that in the transformation and use it to apply a different CSS class, like Brenden suggests. Something like
<div class="category-<%# Eval("EventCategory") %>">
Again depending on your needs, you may want to expand
EventLocation with proper address fields (address, city, state, ZIP) to take advantage of the Google Maps webpart on your event listing page. The stock
BookingEvent was too sparse for our site, and we about doubled the number of fields with a new page type.
Not sure about the IMIS side, but maybe there's an RSS, webservice or API you could leverage in Kentico.