in you template folder>>css>>light.css
Find this
.background {
position:absolute;
width:100%;
height:130px;
background:url(../images/light.png) 0 -384px repeat-x !Important;
}
With this.background {
position:absolute;
width:100%;
height:130px;
background-color: #FFF !Important;
}
You can also choose to just delete the line marked !Important as the default background is white.
You might not want to edit any png as the background in afterburner uses sprites.
That is all different backgrounds are in the same image(even the logo is in the same image) that is called light.png.
The using background position only the particular background appears, so I would not advise editing any images.
To get an idea take a look at light.png below
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As you can see not only main background but also the logo and even the pdf icons are in the same image.(thats what makes loading really fast)