Board Thread:Support Requests - Getting Technical/@comment-38051428-20190106030930/@comment-9605025-20190106032620

You don't need custom CSS for this; read Help:Theme_Designer.

I don't understand why you can't post a link. What kind of error is it giving you? Have you tried reading Help:Links?

You can get to the background's file page by going to "File:Wiki-background". On that page is a link to the "original file". Alternatively, you can go to "Special:FilePath/Wiki-background".