Although, the website link you have given seems to be incorrect and not opening.
However, you can use !important rule with the css like below:-

body.page-id-1,body.page-id-2,body.page-id-3 {
background-color: green !important;
}

Hope this will help you!!

The problem is probably that the page IDs are incorrect. You can use your browser’s developer tools to view the body tag and see which classes are on the body for the particular page you want to modify. This is information on Chrome’s DevTools, but all browsers have them: https://developers.google.com/web/tools/chrome-devtools/

body { background-color: red; }

.page-id-1 body, .page-id-2 body , .page-id-3 body { 
    background-color: green!important; 
}