CSS Background Image Not Displaying

According to your CSS file path, I will suppose it is at the same directory with your HTML page, you have to change the url as follows:

body { background: url(img/debut_dark.png) repeat 0 0; }

You should use like this:

        body { 
            background: url("img/debut_dark.png") repeat 0 0;
        }

        body { 
            background: url("../img/debut_dark.png") repeat 0 0;
        }

        body { 
            background-image: url("../img/debut_dark.png") repeat 0 0;
        }

or try Inspecting CSS Rules using Firefox Firebug tool.