There are a couple easy ways to do this:
1. Use a body class to target your CSS: Each page/post in wordpress has a unique class added to the body tag which includes the ID of the page. You can target that CSS class to change the background color on only that page.
2. Use Inline CSS (recommended): Use the HTML block to add inline CSS to the page/post itself. Simply add your CSS between <style></style> tags. Like this:
By pure luck, I’ve actually written a tutorial on this on my site: https://www.buildthatwebsite.com/change-background-color-specific-page