So, I've found other posts on backgrounds. Thing is, I followed the instructions exactly. (I think)
I created the CSS file, but how do I put it into my description to create a background?
This is what I uploaded in the CSS file:

@media (min-width:768px) {
body {
background: url('https://i.pinimg.com/originals/14/5c/e6/145ce6b3fa7509f6b2440ad7719ec42e.jpg') center center fixed !important;
background-repeat: no-repeat;
background-size: 100%;
}
}

@media (max-width: 768px) {
body {
background: url('https://i.pinimg.com/originals/14/5c/e6/145ce6b3fa7509f6b2440ad7719ec42e.jpg') center center fixed !important;
background-repeat: no-repeat;
background-size: 100%;
width: auto !important;
min-width: 600px !important;
}
}