It's a special namespace, and all pages in this namespace have the class ns-112 in their body section. Here's the logo, from MediaWiki:Wikia.css:
.ns-112 #WikiHeader h1.wordmark img {zoom:1; opacity:0; filter: alpha(opacity=0); -ms-filter: "alpha(opacity=0)"; -khtml-opacity:0; -moz-opacity:0;} .ns-113 #WikiHeader h1.wordmark img {zoom:1; opacity:0; filter: alpha(opacity=0); -ms-filter: "alpha(opacity=0)"; -khtml-opacity:0; -moz-opacity:0;} .ns-114 #WikiHeader h1.wordmark img {zoom:1; opacity:0; filter: alpha(opacity=0); -ms-filter: "alpha(opacity=0)"; -khtml-opacity:0; -moz-opacity:0;} .ns-115 #WikiHeader h1.wordmark img {zoom:1; opacity:0; filter: alpha(opacity=0); -ms-filter: "alpha(opacity=0)"; -khtml-opacity:0; -moz-opacity:0;}