If possible, the best way is to provide each page with a unique or useful class and / or body element identifier. So, for example, one page:
<body class="bg-custom">
and the other -
<body class="bg-custom-right">
Then in your CSS you can change it like this:
.bg-custom #selector { background-img: url(...);}
.bg-custom-right #selector { background-img: url(...);}
#selector - , , () , CSS HTML . ( , , .)