I think you’ve misunderstood me.
You are currently using this CSS to add the background image on various pages:
body.page-id-60, body.page-id-6633, body.page-id-10, body.page-id-6500, body.blog, body.page-id-6502, body.page-id-6505, body.page-id-6511 {
background-image: url(http://new.nimblethimble.com/wp-content/uploads/2019/05/white_quilit_background_levels.jpg);
}
If you also want it on single posts, that basically means you want that background image on every page and post of your site. In that case, simply upload it through the customizer as I mentioned before and you can remove the CSS.
Let me know if this is clear π