【background】
<!DOCTYPE html>
<html>
<head>
<style>
#test {
background: red;
width: 30px;
height: 30px;
}
</style>
</head>
<body>
<div id="test"></div>
<script>
var a = document.getElementById('test');
a.style.background = "#00f";
</script>
</body>
</html>
