How do I change the calendar background color?

One way to change the calendar background color is to choose a different form theme, at Edit Form → Settings → Style → Global.

If you want to keep the current form theme you can change the color of just the calendar background by going to Forms → Settings → Custom CSS & JS → Custom CSS and adding this CSS:

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.quform-datepicker .k-calendar .k-header,
.quform-datepicker .k-calendar .k-footer,
.quform-datepicker .k-calendar .k-header .k-state-hover {
    background-color: #123456 !important;
}

.quform-datepicker .k-calendar .k-header,
.quform-datepicker .k-calendar .k-footer,
.quform-datepicker .k-calendar .k-header .k-state-hover {
	background-color: #123456 !important;
}

Replace #123456 with the desired hex color.