From what I know, first theme to use window transparency.
This requires my Transparency Patcher, without it the theme will look very black.
You can set a semi-transparent background using this css I found on the internet, which you should throw in your Custom CSS window:
body::after {
content: "";
background: url(https://i.redd.it/ljf70pg0z76y.jpg);
opacity: 0.3;
top: 0;
left: 0;
bottom: 0;
right: 0;
position: absolute;
z-index: -1;
}