Within the latest version of Wordpress it offers a superior the chance to see a preview of the items your website would seem like utilizing a different heme. You essentially just click the theme also it gets control the screen and you've got an opportunity to activate it or close it (and go back to the prior screen, that is grayed out without anyone's knowledge). I've come across an identical technique utilized on numerous websites lately for display images too.

I am wondering what technology/code they will use to get this done?

It's free - make use of the source, Luke.

Try looking in wordpress-admin/js/theme-preview.js