.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'.