<head>
<link rel="stylesheet" type="text/css" href="theme.css">
</head>
Actually href is a html attribute, which is used for passing page url to the a tag.
Example
<a href="url">Click here</a>
<a href="mailto:mhmd.shrydh1996@gmail.com">Send me Email</a>
<!-- This is how you use a <a> tag in html -->
<a class="nav-link" href="Story.html">Story</a>
<!-- href is an "attribute" in html used in <a> tag etc..-->
<a href="https://dg-games.godaddysites.com">WEBSITE</a>
<link rel="stylesheet" href="./styles.css">