Change colour of cell in CSS style sheet

Hi

I have been trying to change the colour of a day cell on the grid calender, month view, when an event is showing. Even though my category is white, the event still shows up with grey text on a black background in the cell. It is difficult to see the text, so I would rather the cells on the calender always be white or a very light grey.

I am very new to CSS so I am really not sure what I am doing. I have tried to edit some of the colours in the style sheet but it just resets every time I leave the page.

Any help would be much appreciated! Thank you.

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