How to change the background color of a non-event grid cell based on an office work schedule

FullCalendar provides a callback function named "eventContent" where the developer can basically hijack the HTML generation and return it back to FC. However, I cannot find any way to do the same thing with non-event grid cells. I need to change the background color of non-work grid cells based on an office's work schedule. Does anyone know if this is possible?

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