Board Thread:Support Requests - Designing Your Wiki/@comment-38546904-20200112205020/@comment-9605025-20200112225847

The CSS in that other thread is just to get the background to show consistently, not to change how it is shown. With that image size, you won't be able to stretch it to fit the screen while maintaining a good appearance. I would recommend tiling instead. If you want to have it fit the width of the screen without getting an annoying lines down the middle, it will need to be at least 2000px wide (it is currently 640px wide). For more information, you can read Help:Theme_Designer and Help:Background.