You might want to have a global CSS file which you might need to reference from your README to have consistent style for the documents. You can refer to the lib directory using the path "lib/...". The system will resolve this path to the right directory.
<!DOCTYPE html>
<html>
<head>
<link rel="stylesheet" href="lib/public/styles.css">
</head>
<body>
<h1>This is README heading</h1>
<p>This is a paragraph.</p>
</body>
</html>