Background Image

Hi,

I have the following problem:

I want to set my website background with an ~1200x800px image in the center of the screen. Setting up the style.css for

body.custom-background {
background-position: center center;
}

doesn’t work for this. Meanwhile I looked up with firebug for firefox. For my body I get the following code contained in a file (link from firebug shows mainpage from my website) in my root directory, which I can’t find in any file:

body.custom-background {
    background-attachment: fixed;
    background-color: #f4f4f4;
    background-image: url("xxx");
    background-position: center top;   <------
    background-repeat: no-repeat;
}

Can anyone help me?

Thanks a lot