<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
</style>
<link id="CSSsource" href="light.css" rel="stylesheet" />
<script>
var n = 0;
function changeStyle()
{
document.getElementById("CSSsource").href = ["light.css","dark.css"][++n%2]
}
</script>
</head>
<body>
<button type="button" onclick="changeStyle();" >изменить css</button>
</body>
</html>