.flatpickr-calendar{
    font-size:16px;
}

.flatpickr-day{
    width:48px;
    height:48px;
    line-height:48px;
}

.flatpickr-weekday{
    font-size:15px;
}

.flatpickr-current-month{
    font-size:18px;
    font-weight:bold;
}

/* Días ocupados */
.flatpickr-day.flatpickr-disabled,
.flatpickr-day.flatpickr-disabled:hover{
    background: #dc3545 !important;
    border-color: #dc3545 !important;
    color: #fff !important;
    cursor: not-allowed;
    opacity: 1 !important;
}

/* Si quieres que el número se vea en negrita */
.flatpickr-day.flatpickr-disabled{
    font-weight: bold;
}