From the menu go to: Insert > CSS > Remote Link.
The "The <link> tag allows you to use one style sheet for all HTML documents. The <link> tag is inserted inside the head section
Example:
<html>
<head>
<title> Your Title Here </title>
<link rel="stylesheet" type="text/css" href="style.css">"
</head>
<body>
</body>
</html>
