How to randomly assign class from array

Question

I am trying to randomly add a class from a list preset in jQuery to an element e.g

Classes available: Theme1, Theme2, Theme3

var NumClasses = 3
function randOrd() {
    return (Math.round(Math.random())-0.NumClasses); 
}
$(document).ready(function() {
    var klasses = [ 'Theme1', 'Theme2', 'Theme3' ];
    klasses.sort( randOrd );
    $("#main-body-header").addClass(klasses[i]);
});