fediversity.eu/themes/hugoplate/layouts/partials/components/theme-switcher.html

<!-- theme switcher -->
{{ $class := .Class }} {{ if site.Params.theme_switcher }}
<div class="theme-switcher {{ $class }}">
  <input id="theme-switcher" data-theme-switcher type="checkbox" />
  <label for="theme-switcher">
    <span class="sr-only">theme switcher</span>
    <span>
      <!-- sun -->
      <svg
        class="absolute left-1/2 top-1/2 -translate-x-1/2 -translate-y-1/2 z-10 opacity-100 dark:opacity-0"
        viewBox="0 0 56 56"
        fill="#fff"
        height="16"
        width="16"
      >
        <path
          d="M30 4.6c0-1-.9-2-2-2a2 2 0 0 0-2 2v5c0 1 .9 2 2 2s2-1 2-2Zm9.6 9a2 2 0 0 0 0 2.8c.8.8 2 .8 2.9 0L46 13a2 2 0 0 0 0-2.9 2 2 0 0 0-3 0Zm-26 2.8c.7.8 2 .8 2.8 0 .8-.7.8-2 0-2.9L13 10c-.7-.7-2-.8-2.9 0-.7.8-.7 2.1 0 3ZM28 16a12 12 0 0 0-12 12 12 12 0 0 0 12 12 12 12 0 0 0 12-12 12 12 0 0 0-12-12Zm23.3 14c1.1 0 2-.9 2-2s-.9-2-2-2h-4.9a2 2 0 0 0-2 2c0 1.1 1 2 2 2ZM4.7 26a2 2 0 0 0-2 2c0 1.1.9 2 2 2h4.9c1 0 2-.9 2-2s-1-2-2-2Zm37.8 13.6a2 2 0 0 0-3 0 2 2 0 0 0 0 2.9l3.6 3.5a2 2 0 0 0 2.9 0c.8-.8.8-2.1 0-3ZM10 43.1a2 2 0 0 0 0 2.9c.8.7 2.1.8 3 0l3.4-3.5c.8-.8.8-2.1 0-2.9-.8-.8-2-.8-2.9 0Zm20 3.4c0-1.1-.9-2-2-2a2 2 0 0 0-2 2v4.9c0 1 .9 2 2 2s2-1 2-2Z"
        />
      </svg>
      <!-- moon -->
      <svg
        class="absolute left-1/2 top-1/2 -translate-x-1/2 -translate-y-1/2 z-10 opacity-0 dark:opacity-100"
        viewBox="0 0 24 24"
        fill="none"
        height="16"
        width="16"
      >
        <path
          fill="#000"
          fill-rule="evenodd"
          clip-rule="evenodd"
          d="M8.2 2.2c1-.4 2 .6 1.6 1.5-1 3-.4 6.4 1.8 8.7a8.4 8.4 0 0 0 8.7 1.8c1-.3 2 .5 1.5 1.5v.1a10.3 10.3 0 0 1-9.4 6.2A10.3 10.3 0 0 1 3.2 6.7c1-2 2.9-3.5 4.9-4.4Z"
        />
      </svg>
    </span>
  </label>
</div>

<!-- theme switcher -->
<script>
  var darkMode = {{if eq site.Params.theme_default "dark"}}true{{else}}false{{end}};

  {{ if eq site.Params.theme_default "system" }}
  if (window.matchMedia("(prefers-color-scheme: dark)").matches){darkMode = true}
  {{ end }}

  if (localStorage.getItem("theme") === "dark"){darkMode = true}
  else if (localStorage.getItem("theme") === "light"){darkMode = false}
  if (darkMode){document.documentElement.classList.toggle("dark")}
  var themeSwitch = document.querySelectorAll("[data-theme-switcher]");

  document.addEventListener("DOMContentLoaded", () => {
    [].forEach.call(themeSwitch, function (ts) {
      ts.checked = darkMode ? true : false;
      ts.addEventListener("click", () => {
        document.documentElement.classList.toggle("dark");
        localStorage.setItem(
          "theme",
          document.documentElement.classList.contains("dark") ? "dark" : "light"
        );
      });
    });
  });
</script>
{{ end }}