.selected_aq{
background:#47D6EB !important;
color : #fff;
}
<select id="abc" multiple="multiple">
<option value="Customer 1" class="selected_aq">Customer 1</option>
<option value="Customer 1" class="selected_aq">Customer 1</option>
</select >
 for (x=0;x<list.options.length;x++){
         if (list.options[x].selected){
             $(list.options[x]).addClass('selected_aq');
          }
Because of 'multiple' attribute background color gets changed to grey but only for last selected 'option'.
 
     
     
    