/m/kbinStyles seems like what you'd want, it's not super populated but I'm sure there's some knowledgeable people who could help you out. If you don't find help there you can also feel free to message me since I have some CSS knowledge.
My bad, you need to remove the quotes from around the URL, it doesn't like those for some reason. So it'll be url(https://imgur.com/9Ns6RVS.png)
No worries, you need to include the pound sign before middle. So
#middle { background-image: url('https://imgur.com/9Ns6RVS.png'); background-repeat: no-repeat; background-size: cover; }
should work.
Figured it out, you need to target #middle instead of body for the background to appear :)