In one of my 'random wiki' adventures I happen to land on the Ghost In The Shell wiki, and how the background shows through the WikiaPage blows my mind. I knew CSS must play a part in this, but a search through its very lengthy common.css for 'opacity' doesn't return any result. A quick look at w3schools too didn't explain how to only make the background translucent and not the content (text, etc.)
- Go to Special:ThemeDesigner on your wiki. In the second tab (Customize), the last coloured box affects the article background. The slider underneath this does the transparency. Hope this helps!