The grey background is made with a grey shaded background image on the body.
You can either change that to your own image by adding the rule below in MAin Options > Fonts & Custom > Custom CSS Rule box
body {background-image: url(imageURL);}
Or you can use a gradient CSS rule as shown below with your own start and end color if you want a uniform gradient from top to bottom
body {
background-image: -webkit-gradient(linear, left top, left bottom, from(#444444), to(#999999));
background-image: -webkit-linear-gradient(top, #444444, #999999);
background-image: -moz-linear-gradient(top, #444444, #999999);
background-image: -o-linear-gradient(top, #444444, #999999);
background-image: -ms-linear-gradient(top, #444444, #999999);
background-image: linear-gradient(to bottom, #444444, #999999);
filter:prodig:DXImageTransform.Microsoft.gradient(startColorstr=’#444444′,endColorstr=’#999999′);
}