I disagree, it's a question of authoring freedom. We have a fairly recent "prefers-color-scheme: dark" media query for this which further proves my point, that it's either black or white by default.
If you, the author of the page decide to respect it then it's fine, but I'd be a bit mad if the browser decided to override my page design to an arbitrary grey shade if I don't declare a background color for the body.
The prefers-color-scheme media query can return any of 'light', 'dark' or 'no-preference' (the default). There's no real default for "light" design except as may be provided by the page author, or perhaps by the user herself via setting such a preference.
If you, the author of the page decide to respect it then it's fine, but I'd be a bit mad if the browser decided to override my page design to an arbitrary grey shade if I don't declare a background color for the body.