Go to the particular page. Right-click the page then select “View Source” / “View Page Source”. Find the <body> tag to get the unique page class.
Example:
<body class="page page-id-8 custom-background">
In the example, the unique page class is “page-id-8”
You’ll then add something like this on the Custom CSS option:
.page-id-8 article.single-view {background-color: #A3FC90;}