Add an extra class to views exposed filter “select” if default option is selected
Change the selector and class name to your needs, but this should work.

(function ($) {

  $("select")
    .change(function() {
      if ($(this).val() === $(this).find('option[selected]').val()) {
        $(this).addClass('default-value-selected');
      }
      else {
        $(this).removeClass('default-value-selected');
      }
    })
    .trigger("change");

})(jQuery);