In the latest version of Wordpress it gives you the opportunity to view a preview of what your site would look like using a different heme. You basically just click on the theme and it takes over the screen and you have a chance to activate it or close it (and return to the previous screen, which is grayed out in the background). I have seen a similar technique used on a number of websites recently for display images as well.
I'm wondering what technology/code they use to do this?