Try this:

function changeScheme(){
   var sel = $('#HOT_ColorsTheme').val();
   if (sel=='A') {
      $(".Today").css({'background' : '#fe0002', 'color' : '#000000'});
   }
   if (sel=='B') {
      $(".Today").css({'background' : '#dedede', 'color' : '#000000'});
   }
}

$(document).ready(function(){
   changeScheme();
   $("#HOT_ColorsTheme").on('change', changeScheme());
});

you can just write a single additional line at the end of $(function(){ ---- }); (i.e. document ready code)

$(function(){
--
----- your other js code if any
--
    // triggering the change event on the select by referring its id
    $('#HOT_ColorsTheme').trigger('change');
});

Assuming your color changing code is working properly, It will just trigger the change event, when your document will be ready, and when the change event is triggered, your above function is there to handle the event.