MediaWiki:Common.css

/* CSS placed here will be applied to all skins */
.mw-body-content {
    max-width: 1250px;
}

.mw-panel {
    width: 240px; /* default is ~160px, adjust as needed */
}

/* Adjust content area to not overlap */
#content, #left-navigation {
    margin-left: 260px; /* must be >= sidebar width */
}