An External CSS:-
An external style sheet is used to define the style for many HTML pages.With an external style sheet, you can change the look of an entire web site, by changing one file!
To use an external style sheet, add a link to it in the <head> section of the HTML page:
Example
<!DOCTYPE html>
<html>
<head>
<link rel="stylesheet" href="styles.css">
</head>
<body>
<h1>This is a heading</h1>
<p>This is a paragraph.</p>
</body>
</html>