<head>
<title></title>
<style type="text/css">
.day
{
background-color:White;
}
.night
{
background-color:Black;
}
</style>
<script type="text/javascript">
function SwitchLight() {
var btn = document.getElementByIdx_x("btnChange");
if (document.body.className == "night") {
document.body.className = "day";
btn.value = "关灯";
}
else if (document.body.className == "day") {
document.body.className = "night";
btn.value = "开灯";
}
}
</script>
</head>
<body class="night">
<input id="btnChange" type="button" value="开灯" onclick="SwitchLight()" />
</body>
