Board Thread:Support Requests - Designing Your Wiki/@comment-24530269-20180216190248/@comment-9605025-20180218175746

Unfortunately, you cannot target an entire page based on which tab is active (at least, not with CSS alone). The reason is that anchors and query strings are not incorporated into a page's class. Aside from efficiency and page caching, there is also a third really good reason the particular syntax in your example isn't used. In CSS,  is a reserved character used to make selections based on id. Similar to how  selects elements that have ,   selects elements that have. So something such as  would select elements that have.

As for why  isn't working, it is because Wikia minifies (concatenates) site CSS. Think of it as creating a new file by copy-and-pasting Common.css and then Wikia.css. This means that the  at the top of Wikia.css is actually in the middle of the style-sheet that the browser actually receives. Since  must be at the top of whichever style-sheet it is in, the one in Wikia.css is ignored. The same is done (separately) for Wikia's global CSS and personal CSS. For example, personal CSS is combined in the following order:
 * global.css
 * common.css
 * .css