MediaWiki:Common.css

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/* CSS placed here will be applied to all skins */
/* Standard infobox styling */
table.infobox {
    float: right;
    clear: right;
    margin: 0 0 1em 1em;
    width: auto;
    border: 1px solid #aaa;
    background-color: #f9f9f9;
    font-size: 90%;
    line-height: 1.4em;
}
.infobox {
    width: 300px;       /* or whatever fixed width you want */
    max-width: 100%;    /* keeps it responsive on smaller screens */
    float: right;       /* or left, if you want it on the side */
    margin: 0 0 1em 1em; /* spacing around it */
}