MediaWiki:Common.css

table.calendar,
table.simplecalendar {
    width: 100% !important;
    border-collapse: collapse !important;
    font-family: sans-serif !important;
    font-size: 0.95em !important;
    margin: 1em 0 !important;
}
.mw-parser-output table.calendar {
    width: 100% !important;
    font-size: 1.1em !important;
    table-layout: fixed !important;
}
.calendar th,
.calendar td,
.simplecalendar th,
.simplecalendar td {
    border: 1px solid #ccc !important;
    padding: 0.75em !important;
    text-align: center !important;
    vertical-align: top !important;
    min-width: 90px !important;
}

.calendar td a,
.simplecalendar td a {
    display: block !important;
    color: #0366d6 !important;
    text-decoration: none !important;
    font-weight: bold !important;
    padding: 0.3em !important;
}

.calendar td a:hover,
.simplecalendar td a:hover {
    background-color: #f5f5f5 !important;
    border-radius: 4px !important;
}