The code you sent also seems to work fine. If I had to guess, I would think it's the browser you're using. I did a little googling on how extensions work with Vivaldi. Seems you just use Chrome extensions like normal, but some of them do have conflicts. Stylus appears to be one of them.

For now, go into your layout's code. At the top under body, you'll see: background: url("BACKGROUND_IMG");

Add an "!important" in there so that it looks like: background: url("BACKGROUND_IMG") !important;

See if that works for the time being. Either way, I'm going to try working on making these more compatible for other browsers, but there may not be anything I can do in this case. Sorry. :(