app/assets/javascripts/cdx/admin/shared/theme.js
Cdx.ready(function () {
$('.theme-selector').on('select2:select', function (e) {
var theme = e.params.data.id;
var $body = $('body');
$body.removeClass(function (index, className) {
return className.indexOf('skin-') >= 0 ? className : ''
});
$body.addClass('skin-'+theme);
});
});