MediaWiki:Common.css

/* CSS placed here will be applied to all skins */
body {
    background: url("/images/wiki_background.png") no-repeat center center fixed;
    background-size: cover;
}
/* Wrap the sidebar (all portlets) in a white box */
#mw-panel {
    background-color: #ffffff;   /* White background */
    padding: 15px;
    margin: 10px;
    border-radius: 10px;
    box-shadow: 0 2px 8px rgba(0,0,0,0.2);  /* Optional soft shadow */
    border: 1px solid #ccc;
}