how to link to a css file in html

CSS

    <head>

       
      <link rel="stylesheet" type="text/css" href="theme.css">

    </head>
  <head>

       
      <link rel="stylesheet" type="text/css" href="theme.css">

    </head><!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>

Source