Actually in this instance it isn't actually on top of anything, its just an effect. You can see the background image here: http://www.kriesi.at/wp-content/themes/kriesi_at/images/skin1/stuff_bg_less.jpg
There are lots of different ways in which you can make it appear as if a background image is spilling over on top of your content area and this is one of the most simple.
Another is to use z-index with a div that literally floats on top of your site and uses a semi transparent png file so that it is layered on your site.
Its definitely possible, but if you aren't already comfortable with editing html and css or don't want to spend a few days implementing then potentially bug fixing (specially for IE) you can always look into a Freelance web developer to help you get something like this up and running without too much cost.
Hope that helps :)