Overleaf/services/web/public/js/ace/ext/static.css

.ace_static_highlight {
   font-family: 'Monaco', 'Menlo', 'Ubuntu Mono', 'Consolas', 'source-code-pro', 'Droid Sans Mono', monospace;
   font-size: 12px;
}

.ace_static_highlight .ace_gutter {
    width: 25px !important;
    display: block;
    float: left;
    text-align: right;
    padding: 0 3px 0 0;
    margin-right: 3px;
    position: static !important;
}

.ace_static_highlight .ace_line { clear: both; }

.ace_static_highlight .ace_gutter-cell {
  -moz-user-select: -moz-none;
  -khtml-user-select: none;
  -webkit-user-select: none;
  user-select: none;
}


.ace_static_highlight .ace_gutter-cell:before {
    content: counter(ace_line, decimal);
    counter-increment: ace_line;
}
.ace_static_highlight {
    counter-reset: ace_line;
}