background for primary menu

Hi, I'd like to modify my primary menu. I want to make it a full page width stripe instead of tabs-form as it is now. I think the graphic will describe it the best: http://www.loter.pl/images/example-menu.jpg

I've been trying to put image url in
#primary-menu-wrapper {

Didnt work.

Any ideas how to approach this? Will be greatful for any help.