ButserFarm

I've been testing the calendar block here: https://www.butserancientfarm.co.uk/new-page-4

I'm trying to create a calendar view that is colour coded to differentiate between types of event, like in this screenshot. I know CSS will allow me to make all event days the same colour, but I need multiple colours. I've tried cheating by giving the events a header image that's just the colour I'd like them to appear on the calendar, and this sort of works, but I'd ideally like to be able to use a slimline calendar below 600px and I can't figure out how to force the thumbnail to appear on that calendar. Does anyone know either how I can use CSS (and presumably Javascript) to customise date colour on the calendar by event tags, or how I can force an event thumbnail to appear on a slimline calendar? Thanks. Our site is here https://www.butserancientfarm.co.uk/whats-on