linx-server/static/css/highlight/lines.css

.linenumbers {
    border-left: 1px solid #ccc;
    margin-left: 36px;
    position: relative;
}

.linenumbers code {
    counter-reset: linenumbers;
}

.linenumbers code div:before {
    content: counter(linenumbers);
    counter-increment: linenumbers;
    left: -36px;
    width: 28px;
    position: absolute;

    text-align: right;
    padding-right: 5px;
    color: #999;
    overflow-wrap: normal;

    -webkit-touch-callout: none;
    -webkit-user-select: none;
    -khtml-user-select: none;
    -moz-user-select: none;
    -ms-user-select: none;
    user-select: none;
}