Board Thread:Support Requests - Designing Your Wiki/@comment-24530269-20171217160822/@comment-9605025-20171217175117

To get right to the answer, the following is all you should need. However, it appears that the image is not loaded properly. You are not the only user to have this issue and I suspect Wikia may be having issues with their image server. - A few additional notes if you care to know.
 * 1) Using ".background-dynamic" is what caused the issue. The reason is that the page body did not include that in the list of classes. Because your selector required all 3 classes to be possessed by an element, the body was not selected and the background was not applied.
 * 2) "background-attachment: fixed;" is redundant. That is the value already set by the default CSS.
 * 3) Using "!important" is generally not a good idea is it prevents both preceding and proceeding CSS. This means that personal CSS will be overridden by site CSS.
 * 4) If you want to apply the background just to oasis, you can put this CSS on MediaWiki:Wikia.css instead and ditch ".skin-oasis".
 * 5) In terms of the image URL, what I have should be enough. However, the extra parts (including the "?cb=#####") shouldn't hurt. The only time you need to use the extra parts is if a new version of the image has been uploaded but you want to still use the old version.
 * 6) When linking to another Wikia community, you can use interwiki links instead of the full URL. For an example, see my link in #4.