Here a working code with jquery:
<html>
<head>
<script type="text/javascript" src="http://code.jquery.com/jquery-3.1.1.min.js"></script>
</head>
<body class="">
<div class="form-group">
<button class="button">your button</button>
</div>
<script>
var className = "theclass";
jQuery('.button').on( 'click', function(){
localStorage.setItem('className', className);
jQuery('body').addClass(className);
});
// Retrieve the class name from storage
var retrievedClassName = localStorage.getItem('className');
jQuery('body').addClass(retrievedClassName);
</script>
</body>
</html>
And here without:
<html>
<head></head>
<body id="mybigbody" class="">
<div class="form-group">
<button id="button">your button</button>
</div>
<script>
var className = "theclass";
var body = document.getElementById("mybigbody");
document.getElementById('button').onclick = function(e) {
localStorage.setItem('className', className);
body.setAttribute("class", className);
e.stopPropagation();
}
// Retrieve the class name from storage
var retrievedClassName = localStorage.getItem('className');
if(retrievedClassName)
body.setAttribute("class", retrievedClassName);
</script>
</body>
</html>