Hi, thanks for your interest in doing more with the CATEGORIES field. So far I’ve only bothered to provide minimal support for it since, as you noted, most of the major providers, frustratingly, do not use the field.
While there is no automatic CSS styling for categories, the data does get included in the HTML output if the feed actually contains them. This is handled via the data-categories attribute. The value inserted into that attribute is the exact text of the CATEGORIES field from the feed.
So the key to using this in CSS is properly formatting the selectors. You’ll want to use wildcards in case the event has multiple categories. For instance, if your category is “Meetings” then your CSS selector would be:
.event[data-categories*="Meetings"]
If any aspects of this CSS selector structure are unfamiliar, here’s an online reference:
https://www.w3schools.com/cssref/css_selectors.php