Remove color background Mac Calendar Day view color

Change a calendar’s color

1. Go to the Calendar app on your Mac.
2. Control-click the calendar’s name in the calendar list.
3. If you don’t see the calendar list on the left, choose View > Show Calendar List.
4. 
5. Click a preset color to select it, or choose Custom Color to select a color from the Colors window.

Note: You can’t change the color of some calendars, such as the Birthdays calendar and the Siri Suggestions calendar.

Won't show a chage until you unselect that entry.:)

Whew, sorry to say I have no answer for you.

I'll see if I can fid somebody that can.