Unless you entered some custom CSS somewhere, this looks like an old WordPress CSS for the overall background color.
There was a bug at one point that was fixed, so I am not sure why that rule would still be there.
@weaver, do you know what can cause the Inline CSS rule below to be on a site
<style type=”text/css” id=”custom-background-css”>
body.custom-background { background-color: #604035; }
</style>
@pietertje in the mean time you can override that rule by adding the rule below too the theme Global Custom CSS Rule box
body.custom-background { background-color: #506BE0 !important;}