MediaWiki:Common.css

/* CSS placed here will be applied to all skins */ position:center; background-image: url('https://static.wikiforge.net/sherlockwikitide/d/df/Sherlock-cast-i34913.jpg'); background-repeat: no-repeat; height: 600px; width: 1050px; background-size: 85% }
 * 1) image-box {