Изменить класс white на black в “select” js или jquery

$("#sel").change(function(){ 
  $("div").removeClass("white black").addClass($(this).val()); 
}); 
$("#sel").change();

.white{ color:red; } 
.black{ color:green; }

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script> 
<select id="sel" name="sel" class="theme"> 
  <option value="white">white</option> 
  <option value="black">black</option> 
</select> 
 
<div> main </div> 
<div> content1 </div> 
<div> content2 </div>

.white{ 
  color:red; 
} 
.black{ 
  color:green; 
}

<select name="sel" class="theme" onchange="sel.className = this.value"> 
  <option value="white">white</option> 
  <option value="black">black</option> 
</select> 
 
<div id="sel" class='white'> content </div>