How add CSS

Three ways for styling html elements:

link external css file (contains css code) to html document
use element <style>
use attribute style of html element

<head>
    <link rel="stylesheet" href="/css/bootstrap.min.css">
  
    <style>
      a mark{
          text-decoration: underline;
          text-decoration-color: #0a5ffe;
      }
    </style>
</head>

<body>
    <div style="width: 100%; margin-top: 50px;">
    ...
    </div>
</body>

Elements and attributes for styling html document are considered obsolete. For more details see CSS

HTML tutorial

How add CSS