html
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Title</title>
<link rel="stylesheet" href="commons.css" />
</head>
<body>
<div class="c1 c2">adsf</div>
</body>
</html>
css
.c1 {
background-color: red;
color: white;
}
.c2 {
font-size: 28px;
color: black;
}