i did something like that in the past, but it didnt really work back then without js, since css alone didnt know about what year was selected.
But now i have found a method to use js, so it should be possible to do now...
creating a mutation observer on the calander, (or just an event lister on the butons/dropdown) that then can find the selected year/month.
would allow you to apply a style for one of the possible 31 days
here were the styles that didnt work back then:
i dont have time to make a js like this right now, if you have some js experience i can point you to some of the examples and you should be able to do this quite easily, if not than you will have to ping me again in a month orso