Here is a script to add an extra class name to a div with an id regardless of what is there now.
function addClassName (id, classname) {
var e;
if (typeof(id) == 'string') {e = this.getElementById(id);}
else {e = id;}
if (!e || typeof e.className != 'string') {
throw "Cannot add class to element " + id;
}
/* Check if the class is already there */
if (!e.className.match(new RegExp('\\b' + classname + '\\b'))) {
e.className += ' ' + classname;
}
}
