Change the background color of a single page

You can use custom styling to customize most structural elements, however the needed styling depends on the page you want this to apply to and which background.

If you mean you want a background color for the section you activated the shadow wrapping on, try the following custom CSS:

.page article.hentry .article-inner {
    background: #eee;
    padding: 1em; /* avoids having the text too close to the edge */
}