.flatpickr-day.flatpickr-disabled,
.flatpickr-day.flatpickr-disabled:hover {
    color: red !important;
}

.flatpickr-day.prevMonthDay.flatpickr-disabled,
.flatpickr-day.prevMonthDay.flatpickr-disabled:hover,
.flatpickr-day.nextMonthDay.flatpickr-disabled,
.flatpickr-day.nextMonthDay.flatpickr-disabled:hover {
    color: rgba(255, 0, 0, 0.3) !important;
}