This will be difficult as you need go through each div element and set their z-index property lower so background appears slightly on top.
Instead of doing that create 2 new div elements and use position absolute to line them up as how you want it. Please note any links will be underneath the background element so it won't be clickable.
See following link for clear instructions on z-index.
http://www.html.net/tutorials/css/lesson15.phpAs this goes into customization I can not offer further help I am afraid.
See you around...