I'm running the latest gnome-calendar from flatpak nightly repos. 

In both week and month view I noticed that in some situations I can't clearly see where the GcalEventWidget ends. For example, a white calendar can't be distinguished from the background on Adwaita:light.

To fix this, I suggest adding a border to GcalEventWidget.

I've set this in the "CSS" section in GtkInspector, which fixes the issue for both light and dark Adwaita version:

event-widget {
	border-width: 1px;
	border-style: dotted;
}

This will also add borders in year view's sidebar, which looks fine for me but you may not like it.