It is great we were able to clarify some things over chat.
My apologies regarding the animation. This is my first time dealing with animations/transitions. I took another look into it an you do indeed want to use transitions, not animations.
As for the thread posts (forums and/or message walls), do you have any examples where this has been done? Based on my investigation, it seems that there isn't enough information in the HTML to target a specific user.
body.img is a selector for the whole contents of a page only if the body element had a class img, which it doesn't. If you wanted to select all images inside of an article, though, you can change that selector to #mw-content-text img. Or you can use something like what Andrewds1021 suggested above if you don't want them to apply to all images in an article.
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 .myclass selects elements that have class="myclass", #myid selects elements that have id="myid". So something such as .some-class#something.another-class would select elements that have class="some-class another-class" id="something".
As for why @import 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 @import at the top of Wikia.css is actually in the middle of the style-sheet that the browser actually receives. Since @import 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:
Well, there are two ways to deal with this. The first is to import Pages.css in Common.css. As you noted, this would load it for all skins; which may not make sense. The second option is to remove everything besides comments, @charset, @import, and empty lines from Common.css. You may ask, "What about the CSS I have in Common.css?" Excellent question! You will need to move it to a separate stylesheet and then import that stylesheet in Common.css.
Can you link to where you are trying out the link CSS? It looks like the CSS is all you should need.
As for the thread posts, yes, it could be done with JS because JS is not limited to the single-pass that CSS is. CSS is applied in a single, linear pass over the HTML; that is why you cannot select parent or predecessor elements.