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🌙 Astro dark mode

Here is a code snippet I frequently use to add dark/light mode to Astro projects.
It nicely avoids FOUC (flash of unstyled content).

Include this in the <head>:

<script is:inline>
  // dark / light mode
  function checkTheme() {
    const canUseLS = typeof localStorage !== "undefined";
    const stored = canUseLS && localStorage.getItem("theme");
    const classList = document.documentElement.classList;
    if (stored) classList.add(stored);
    else {
      const sys = window.matchMedia("(prefers-color-scheme: light)").matches;
      classList.add(sys ? "light" : "dark");
      classList.remove(sys ? "dark" : "light");
      localStorage.setItem("theme", sys ? "light" : "dark");
    }
  }
  checkTheme();
  document.addEventListener("astro:after-swap", checkTheme);
</script>

Place this button somewhere in the <body>

themeToggle.astro:

---
import "./themeToggle.css";
---

<button id="themeToggle">
  <svg width="30px" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24">
    <path
      class="sun"
      fill-rule="evenodd"
      d="M12 17.5a5.5 5.5 0 1 0 0-11 5.5 5.5 0 0 0 0 11zm0 1.5a7 7 0 1 0 0-14 7 7 0 0 0 0 14zm12-7a.8.8 0 0 1-.8.8h-2.4a.8.8 0 0 1 0-1.6h2.4a.8.8 0 0 1 .8.8zM4 12a.8.8 0 0 1-.8.8H.8a.8.8 0 0 1 0-1.6h2.5a.8.8 0 0 1 .8.8zm16.5-8.5a.8.8 0 0 1 0 1l-1.8 1.8a.8.8 0 0 1-1-1l1.7-1.8a.8.8 0 0 1 1 0zM6.3 17.7a.8.8 0 0 1 0 1l-1.7 1.8a.8.8 0 1 1-1-1l1.7-1.8a.8.8 0 0 1 1 0zM12 0a.8.8 0 0 1 .8.8v2.5a.8.8 0 0 1-1.6 0V.8A.8.8 0 0 1 12 0zm0 20a.8.8 0 0 1 .8.8v2.4a.8.8 0 0 1-1.6 0v-2.4a.8.8 0 0 1 .8-.8zM3.5 3.5a.8.8 0 0 1 1 0l1.8 1.8a.8.8 0 1 1-1 1L3.5 4.6a.8.8 0 0 1 0-1zm14.2 14.2a.8.8 0 0 1 1 0l1.8 1.7a.8.8 0 0 1-1 1l-1.8-1.7a.8.8 0 0 1 0-1z"
    ></path>
    <path
      class="moon"
      fill-rule="evenodd"
      d="M16.5 6A10.5 10.5 0 0 1 4.7 16.4 8.5 8.5 0 1 0 16.4 4.7l.1 1.3zm-1.7-2a9 9 0 0 1 .2 2 9 9 0 0 1-11 8.8 9.4 9.4 0 0 1-.8-.3c-.4 0-.8.3-.7.7a10 10 0 0 0 .3.8 10 10 0 0 0 9.2 6 10 10 0 0 0 4-19.2 9.7 9.7 0 0 0-.9-.3c-.3-.1-.7.3-.6.7a9 9 0 0 1 .3.8z"
    ></path>
  </svg>
</button>

<script is:inline>
  function toggleTheme() {
    const element = document.documentElement;
    element.classList.toggle("dark");

    const isDark = element.classList.contains("dark");
    localStorage.setItem("theme", isDark ? "dark" : "light");
  }

  function addClickListener() {
    document
      .getElementById("themeToggle")
      .addEventListener("click", toggleTheme);
  }

  window.addEventListener("DOMContentLoaded", addClickListener);
  document.addEventListener("astro:after-swap", addClickListener);
</script>

themeToggle.css:

#themeToggle {
  border: 0;
  background: none;
  position: absolute;
  top: 1em;
  right: 1em;
}
.sun {
  fill: black;
}
.moon {
  fill: transparent;
}

.dark .sun {
  fill: transparent;
}
.dark .moon {
  fill: white;
}
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