Here’s the CSS for how the background is applied in this theme, I think this is what you’re looking for?
body {
background-color: #f8f8f8;
background-image: url(https://community.example.com/example.jpg);
background-attachment: fixed;
background-size: auto;
}
By default in Discourse the background color is applied to the HTML tag, so this just covers that up entirely.