<html>
<head>
<script src="https://libs.baidu.com/jquery/1.10.2/jquery.min.js">
</script>
<script>
$(document).ready(function(){
$(":selected").css("background-color","red");
});
</script>
</head>
<body>
<form action="">
Name: <input type="text" name="user"><br>
Car:
<select>
<option>Volvo</option>
<option selected>Saab</option>
<option>Mercedes</option>
<option>Audi</option>
</select>
</form>
</body>
</html>