link css with html

CSS
    <head>

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

    </head>
   <!DOCTYPE html>
<html>
<head>
<link rel="stylesheet" type="text/css" 
  href="mystyle.css">
</head>
<body>

<h1>This is a heading</h1>

  <p>This is a paragraph.</p>

</body>
</html>   <link rel="stylesheet" href="PathToYourFile.css">
Source

Also in CSS: