JSFiddle - React, Tailwind, and code Playground

by Superman

HTML

<span id="datepicker"></span>

CSS

.ui-state-active  {      //1.add this
    background: blue !important;
}
.ui-state-active .ui-state-default{   //2. add this
    background: red !important;
}

JavaScript

var unavailableDates = ["28-1-2015","11-1-2015","15-1-2015"];
    function unavailable(date) {
      dmy = date.getDate() + "-" + (date.getMonth()+1) + "-" + date.getFullYear();
      if ($.inArray(dmy, unavailableDates) < 0) {
        return [true,"","Book Now"];
      } else {
        return [false,"ui-state-active","booked"]; //3. edit here like this..
      }
    }

    $(function() {

             $( "#datepicker" ).datepicker({

                    minDate: 0, 
                    maxDate: "+3M +10D",
                    beforeShowDay: unavailable 

                });

              });