okay, so everything except the font-family thing currently under body needs to go under body::before. I can't test it right now because I'm on mobile, but the code should look like this:

@font-face { font-family: "Zpix"; src: url("https://db.onlinewebfonts.com/t/a760859010799a39cb3cf1960073320a.eot"); }

body {
font-family: "Zpix", sans-serif;
}

body::before {
content: '';
position: fixed;
bottom:-150px;
right: -100px;
width: 400px;
height: 400px;
background-image: url(https://files.catbox.moe/wbb6a9.webp);
background-size: contain;
background-repeat: no-repeat;
z-index: 1000;
pointer-events: none;
}

It also looks like the font wasn't loaded there, so if it still doesn't work I'd try removing the quotes in url("...") or replacing it with a version of the same font in TTF format.

this sort of thing is why there is an "Advanced" warning label on the custom CSS box - if you don't know what you're doing you can really break the site. I'd suggest going through an online CSS tutorial (like the one on W3Schools) and familiarizing yourself with your browser's dev tools if you want to try more.