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HTML 

              
                <main>
  <h1>Sélecteur de thème clair/sombre</h1>

  <button class="theme-switcher js-theme-switcher" type="button" aria-pressed="false">
    <span class="visually-hidden">Changer le mode de couleurs</span>
    <span class="theme-icon">
      <svg class="theme-icon-light" aria-hidden="true" width="24" height="24" viewBox="0 0 24 24" fill="transparent" stroke="currentColor" stroke-linecap="round">
        <path fill="currentColor" d="M18 12a6 6 0 1 1-12 0 6 6 0 0 1 12 0Z"/>
        <path fill="currentColor" fill-rule="evenodd" d="M12 1.25c.41 0 .75.34.75.75v1a.75.75 0 0 1-1.5 0V2c0-.41.34-.75.75-.75ZM4.4 4.4c.3-.3.77-.3 1.06 0l.4.4a.75.75 0 0 1-1.07 1.05l-.4-.4a.75.75 0 0 1 0-1.05Zm15.2 0c.3.3.3.77 0 1.06l-.4.4a.75.75 0 0 1-1.05-1.07l.4-.4a.76.76 0 0 1 1.05 0ZM1.25 12c0-.41.34-.75.75-.75h1a.75.75 0 0 1 0 1.5H2a.75.75 0 0 1-.75-.75Zm19 0c0-.41.34-.75.75-.75h1a.75.75 0 0 1 0 1.5h-1a.75.75 0 0 1-.75-.75Zm-2.1 6.15c.3-.3.77-.3 1.06 0l.4.4a.75.75 0 1 1-1.07 1.05l-.4-.4a.75.75 0 0 1 0-1.05Zm-12.3 0c.3.3.3.77 0 1.06l-.4.4a.75.75 0 1 1-1.05-1.07l.4-.4a.74.74 0 0 1 1.05 0Zm6.15 2.1c.41 0 .75.34.75.75v1a.75.75 0 0 1-1.5 0v-1c0-.41.34-.75.75-.75Z" clip-rule="evenodd"/>
      </svg>
      <svg class="theme-icon-dark" aria-hidden="true" width="24" height="24" viewBox="0 0 24 24" fill="transparent" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
        <path d="M21 12.79A9 9 0 1 1 11.21 3 7 7 0 0 0 21 12.79z" />
      </svg>
    </span>
  </button>

  <section>
    <p>Le thème correspond par défaut au thème utilisateur (<code translate="no">@media prefers-color-scheme</code>) ou à la valeur stockée en <code translate="no">localStorage</code>.</p>
    <p>Le sélecteur ajoute ou modifie l'attribut <code translate="no">data-theme</code> de l'élément <code translate="no">html</code> en lui attribuant la valeur "dark" ou "light".</p>
    <p>La valeur est ensuite stockée en <code translate="no">localStorage</code>.</p>
  </section>
</main>
!

CSS 

              
                .theme-switcher {
  --theme-switcher-radius: 40px;
  --theme-switcher-padding: 4px;
  --theme-switcher-outline-color: light-dark(#777, #bbb);
  --theme-switcher-outline-size: 2px;
  --theme-switcher-background: light-dark(#333, #777);
  --theme-switcher-icon-size: 24px;
  --theme-switcher-icon-color: #fff;
  --theme-switcher-icon-background: light-dark(#777, #333);
  --theme-switcher-transition: 0.25s;

  display: inline-flex;
  justify-content: start;
  width: calc((var(--theme-switcher-icon-size)) * 2.3);
  padding: var(--theme-switcher-padding);
  border: 0;
  outline: var(--theme-switcher-outline-size) solid var(--theme-switcher-outline-color);
  border-radius: var(--theme-switcher-radius);
  background-color: var(--theme-switcher-background);
  line-height: 0;
  cursor: pointer;
  
  &::before {
    content: "";
    flex: 0 0 0;
    transition: var(--theme-switcher-transition) flex-grow;
    will-change: flex-grow;
  }

  & * {
    pointer-events: none;
  }

  & > .theme-icon {
    padding: 2px;
    border-radius: 50%;
    color: var(--theme-switcher-icon-color);
    background-color: var(--theme-switcher-icon-background);
    rotate: 0deg;
    transition: var(--theme-switcher-transition) rotate;
  }
  

  & [class^="theme-icon-"] {
    width: var(--theme-switcher-icon-size);
    height: var(--theme-switcher-icon-size);
  }
  
  &:focus-visible {
    outline-color: currentcolor;
    outline-offset: 2px;
  }
  
  &[aria-pressed="false"] {

    & .theme-icon-dark {
      display: none;
    }
  }

  &[aria-pressed="true"] {
    
    & .theme-icon-light {
      display: none;
    }
    
    & > .theme-icon {
      rotate: 240deg;
    }

    &::before {
      flex: 1 0 0;
   }
  }

}

:root {
  color-scheme: light dark;
  --colors-white: #EEEEEE;
  --colors-black: #333333;
  --colors-primary: light-dark(var(--colors-black), var(--colors-white));
  --colors-secondary: light-dark(var(--colors-white), var(--colors-black));
  --font-size-base: 1.2rem;
  --font-weight-normal: 400;
  --font-weight-bold: 700;
  --border-radius-full: 9999px;
}

&[data-theme="light"] {
  color-scheme: light;
}

&[data-theme="dark"] {
  color-scheme: dark;
}

@media (prefers-reduced-motion: reduce) {
  * {
    transition: none !important;
  }
}

html {
  color: var(--colors-primary);
  background-color: var(--colors-secondary);
  font-family: system-ui, sans-serif;
}

body {
  display: grid;
  min-height: 100dvh;
  place-content: start center;
  justify-items: center;
  margin: 0;
  font-size: var(--font-size-base);
}

.visually-hidden {
  position: absolute;
  width: 1px;
  height: 1px;
  margin: -1px;
  padding: 0;
  overflow: hidden;
  clip: rect(0 0 0 0);
  border: 0;
}

a {
  color: inherit;
}

code {
  font-size: inherit;
}

main {
  padding: 4rem;
  margin-inline: auto;
  max-width: 800px;
}


@media (width >= 40rem) {
  main {
    max-width: 1000px;
  }
}
!

JS 

              
                // Encapsulation du code dans une fonction immédiatement invoquée
(function() {
  // Déclaration des variables
  const themeSwitcher = {
    button: document.querySelector('.js-theme-switcher'),
    prefersDark: window.matchMedia('(prefers-color-scheme: dark)'),
    currentTheme: null
  };

  // Initialisation du thème
  function initTheme() {
    if (localStorage.getItem('theme')) {
      themeSwitcher.currentTheme = localStorage.getItem('theme');
    } else if (themeSwitcher.prefersDark.matches) {
      themeSwitcher.currentTheme = 'dark';
    } else {
      themeSwitcher.currentTheme = 'light';
    }
    setTheme(themeSwitcher.currentTheme);
  }

  // Gestion du clic sur le bouton
  themeSwitcher.button.addEventListener('click', function() {
    themeSwitcher.currentTheme = document.documentElement.getAttribute('data-theme') === "dark" ? "light" : "dark";
    setTheme(themeSwitcher.currentTheme);
  });

  // Gestion du changement de préférence système
  themeSwitcher.prefersDark.addEventListener('change', function(event) {
    themeSwitcher.currentTheme = event.matches ? 'dark' : 'light';
    setTheme(themeSwitcher.currentTheme);
  });

  // Fonction pour définir le thème
  function setTheme(theme) {
    const pressed = theme === 'dark' ? 'true' : 'false';
    document.documentElement.setAttribute('data-theme', theme);
    localStorage.setItem('theme', theme);
    themeSwitcher.button.setAttribute('aria-pressed', pressed);
  }

  // Initialisation
  initTheme();
})();
!
999px

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