Unfortunately, this is no way to change the background color of an individual page from within the admin.
You'd need to do this with some custom CSS in the custom.css file.
You can, however, target individual pages (or page types) using this method.
Check out your page's <body> tag in your web browser's Developer Tools -> Page Source for classes that might help in targeting your page.
You might find something like this:
<body class="light type-basic page-12 group-0 body-arial headings-arial ">
You could target this page using the 'page-12' class (each page has a unique page number), e.g:
.page-12 {
background-color: #000000;
}
... or you could target all basic pages as follows:
.type-basic {
background-color: #000000;
}
I hope this points you in the right direction.
However, if you continue to experience difficulties, please post the URL of the web page and let me know what color you'd like it's background to be and I'll see if I can help further.
Thank you..