JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="https://dl.dropboxusercontent.com/s/y4enc3buyzz2m3a/fullcalendar.css">
<script src="https://dl.dropboxusercontent.com/s/93dskqt37q1caxr/fullcalendar.min.js"></script>
<div id='calendar'></div>

CSS

.cellBg {
    background: #000;
    color: #fff;
}

JavaScript

var $calendar = $('#calendar').fullCalendar({
    header: {
        left: 'prev,next today',
        center: 'title',
        right: 'month,basicWeek,basicDay'
    },
    
    defaultView: 'month',

    dayRender: function (date, cell) {
        //var today = new Date();
        /*if (date.getDate() === today.getDate()) {
            cell.css("background-color", "red");
        }*/
         date = new Date();
         date = $.fullCalendar.formatDate(date, 'yyyy-MM-dd');
         $('.fc-day[data-date="'+ date +'"]').addClass('cellBg');
    }
});