flyte,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
</head>
<body>
<style type="text/css">
.bordered{
border: 1px dashed Gray; padding: 5px; height: 100px; width: 100px
}
.bordered:after{
content: "12345"
}
</style>
<div class="bordered"></div>
<script>
document.head.insertAdjacentHTML('beforeend', '<style>.bordered:after{background:#1F9C63}</style>');
</script>
</body>
</html>