<body class="bg_primary color_secondary">
  <main class="bg_primary color_secondary">
    <dkv class="center">
      <button class="js_switch">
        Switch Mode
      </button>
    </dkv>
    <h1 class="color_secondary">L'interMEDIAire</h1>
    <h2 class="color_secondary">Titre 2</h2>
    <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Lorem ipsum dolor sit amet, consectetur adipisicing elit. Quibusdam ipsam delectus sapiente officia temporibus quam dolorum nam iusto asperiores vitae, quia, ratione hic quod vel unde quae tempore a in.Earum sapiente cupiditate ducimus amet quo provident! Adipisci eaque ducimus incidunt tempora ipsa? Iusto, repellat aliquam reprehenderit fugit! Perspiciatis, reprehenderit quisquam excepturi!</p>
    <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Ipsum velit nam illo molestiae earum, distinctio recusandae omnis eius soluta. Saepe asperiores libero modi est eum rem aliquid dolorum laudantium magnam.</p>
    <a href="#">Plus d'infos</a>
    <img src="http://placekitten.com/1080/540" alt="Chat !" />
  </main>
</body>
body,
body[data-theme="dark"] {
  --color_primary: black;
  --color_secondary: rgba(152,38,114,1);
  --grayscale: grayscale(100%);
}

body[data-theme="light"] {
  --color_primary: rgba(152,38,114,1);
  --color_secondary: black;
  --grayscale: grayscale(0%);
}

$blanc:       rgba(255,255,255,1);
$gris2:       rgba(255,255,255,.8);
$padding:     20px 10px;

*,*:after,*:before {
  box-sizing: border-box;
  -moz-box-sizing: border-box;
  -webkit-box-sizing: border-box;
}

body {
  margin: 0px auto;
  padding: $padding;
  max-width: 640px;
  background: var(--color_primary);
  color: var(--color_primary);
  font-size: 1em;
}

main {
  padding: $padding;
}

a {
  color: var(--color_secondary);
  text-decoration: none;
  transition: all .3s ease-in-out;
}

a:hover,
a:active {
  color: $blanc;
}

h1 {
  font-size: 1.6em;
}

h2 {
  margin-top: 12px;
  font-weight: normal;
}

h1,
h2 {
  color: var(--color_secondary);
  font-family: 'Inconsolata', monospace;
}

p {
  color: $gris2;
  line-height: 1.6em;
}

img {
  margin-top: 20px;
  width: 100%;
  height: auto;
  filter: var(--grayscale);
}

button {
  margin-bottom: 24px;
  padding: 0px 12px;
  height: 40px;
  background: var(--color_primary);
  color: var(--color_secondary);
  line-height: 40px;
  border: 1px solid #{var(--color_secondary)};
  cursor: pointer;
}

/*www.lintermediaire.be*/
View Compiled 
const btn = $('.js_switch')
let darkMode = localStorage.getItem("dark-mode")

const enableDarkMode = () => {
  $('body').attr("data-theme", "dark")
  localStorage.setItem("dark-mode", "enabled");
};

const disableDarkMode = () => {
  $('body').attr("data-theme", "light")
  localStorage.setItem("dark-mode", "disabled");
};

if (darkMode === "disabled") {
  disableDarkMode()
}
else {
  enableDarkMode()
}

$(btn).on('click', function(e) {
  darkMode = localStorage.getItem("dark-mode")
  if (darkMode === "disabled") {
    enableDarkMode()
  } else {
    disableDarkMode()
  }
})

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

  1. https://cdnjs.cloudflare.com/ajax/libs/jquery/3.7.1/jquery.min.js