Hey Andreas,
Thanks for reaching out.
I apologize for the delayed response — we took some additional time here to investigate and test the Underscores theme ourselves, to ensure there’s not a bug here.
The Underscores theme is a major part of the WordPress theme standards these days, and is in fact the basis for core themes like Twentyseventeen itself. So we wanted to be absolutely there’s no bugs with that!
I was unable to find any such bugs, and had no issues with applying custom CSS or with having the CSS of the calendar rendering.
Since I could not recreate any issues, and since Underscores is the basis of Twentyseventeen itself, it stands to reason then that something specific and unique to your custom code is the culprit in the issues you are facing.
We are unfortunately not able to help troubleshoot custom code, so you will have to take the reins on investigating your own issues, but hopefully this helps narrow down the search a bit.
— George