MediaWiki:Common.css

/* CSS placed here will be applied to all skins */

body {
    background-color: #000000;
    /* this assumes that your wiki is installed at '/w' in the web root */
    background-image: url('https://uoendurance.com/wiki/images/8/81/Bg1.gif');
    background-repeat: no-repeat;
    background-size: auto;
    max-width: 100%;
    height: auto;
    width: auto\9;
}

#mw-page-base {
  background-image: none;
  background-color: transparent;
}