How to Link Lightning Design System CSS Stylesheet

I'm finishing coding a booking website with HTML/CSS but there's probably a problem with the link tag in the <head>.

Since I have no design skills, I decided to use Lightning Design System for styling the website. But when I launch my site, the CSS does not appear. Here is the <head> tag of the site's home page where I insert the style sheet link:

<head>
    <meta charset="utf-8" />
    <meta http-equiv="x-ua-compatible" content="ie=edge" />
    <!-- Set the viewport width to device width for mobile -->
    <meta name="viewport" content="width=device-width, initial-scale=1" />
    <title>Accueil</title>
    <!-- Link to lightningdesignsystem for page style -->
    <link rel="stylesheet" href="/static/style.css">
    <link rel="stylesheet" type="text/css"
        href="https://www.lightningdesignsystem.com/_next/static/css/1b7615e9db05d7a5f4b8.css" />
    <!-- Jquery calendar used for bookings -->
    <link rel="stylesheet" type="text/css" href="/jquery.datetimepicker.css" />
    <script src="/build/jquery.datetimepicker.full.min.js"></script>
</head>

Please guide me on how to properly link the styles.