Astro

Adding dark mode to your Astro site.

Just like in regular Svelte, we use the class strategy from Tailwind CSS to support dark mode toggling. See the Tailwind CSS documentation for more information.

How you add the dark class to the html element is up to you. In this guide, we'll take a look at enabling dark mode toggling with mode-watcher.

Usage

Create an inline theme script

This script will, in part, keep and track the dark mode value in localStorage and prevent FUOC.

src/pages/index.astro

---
import "$lib/styles/app.css";
---
 
<script is:inline>
  const isBrowser = typeof localStorage !== 'undefined';
  const getThemePreference = () => {
    if (isBrowser && localStorage.getItem('theme')) {
      return localStorage.getItem('theme');
    }
    return window.matchMedia('(prefers-color-scheme: dark)').matches
      ? 'dark' : 'light';
  };
  const isDark = getThemePreference() === 'dark';
  document.documentElement.classList[isDark ? 'add' : 'remove']('dark');
 
  if (isBrowser) {
    const observer = new MutationObserver(() => {
      const isDark = document.documentElement.classList.contains('dark');
      localStorage.setItem('theme', isDark ? 'dark' : 'light');
    });
    observer.observe(document.documentElement, {
      attributes: true,
      attributeFilter: ['class']
    });
  }
</script>
 
<html lang="en">
	<body>
      <h1>Astro</h1>
	</body>
</html>
</script>

Install mode-watcher

pnpm i mode-watcher@0.5.1

npm i mode-watcher@0.5.1

bun install mode-watcher@0.5.1

yarn install mode-watcher@0.5.1

Add the ModeWatcher component

Import the ModeWatcher component and use it in your page with the client:load directive:

src/pages/index.astro

---
import "$lib/styles/app.css";
import { ModeWatcher } from "mode-watcher";
---
 
<!-- inline-script -->
<html lang="en">
	<body>
      <h1>Astro</h1>
      <ModeWatcher client:load />
	</body>
</html>

Create a mode toggle

Create a mode toggle on your site to toggle between light and dark mode:

Light switch

<script lang="ts">
  import SunIcon from "@lucide/svelte/icons/sun";
  import MoonIcon from "@lucide/svelte/icons/moon";
 
  import { toggleMode } from "mode-watcher";
  import { Button } from "$lib/components/ui/button/index.js";
</script>
 
<Button onclick={toggleMode} variant="outline" size="icon">
  <SunIcon
    class="h-[1.2rem] w-[1.2rem] rotate-0 scale-100 !transition-all dark:-rotate-90 dark:scale-0"
  />
  <MoonIcon
    class="absolute h-[1.2rem] w-[1.2rem] rotate-90 scale-0 !transition-all dark:rotate-0 dark:scale-100"
  />
  <span class="sr-only">Toggle theme</span>
</Button>

<script lang="ts">
  import SunIcon from "@lucide/svelte/icons/sun";
  import MoonIcon from "@lucide/svelte/icons/moon";
 
  import { resetMode, setMode } from "mode-watcher";
  import * as DropdownMenu from "$lib/components/ui/dropdown-menu/index.js";
  import { buttonVariants } from "$lib/components/ui/button/index.js";
</script>
 
<DropdownMenu.Root>
  <DropdownMenu.Trigger
    class={buttonVariants({ variant: "outline", size: "icon" })}
  >
    <SunIcon
      class="h-[1.2rem] w-[1.2rem] rotate-0 scale-100 !transition-all dark:-rotate-90 dark:scale-0"
    />
    <MoonIcon
      class="absolute h-[1.2rem] w-[1.2rem] rotate-90 scale-0 !transition-all dark:rotate-0 dark:scale-100"
    />
    <span class="sr-only">Toggle theme</span>
  </DropdownMenu.Trigger>
  <DropdownMenu.Content align="end">
    <DropdownMenu.Item onclick={() => setMode("light")}>Light</DropdownMenu.Item
    >
    <DropdownMenu.Item onclick={() => setMode("dark")}>Dark</DropdownMenu.Item>
    <DropdownMenu.Item onclick={() => resetMode()}>System</DropdownMenu.Item>
  </DropdownMenu.Content>
</DropdownMenu.Root>

Add mode toggle to page

Add the mode toggle to the page (also with the client:load directive):

src/pages/index.astro

---
import "$lib/styles/app.css";
import { ModeWatcher } from "mode-watcher";
import ModeToggle from "$lib/components/mode-toggle.svelte";
---
 
<!-- inline-script -->
<html lang="en">
	<body>
      <h1>Astro</h1>
      <ModeWatcher client:load />
      <ModeToggle client:load />
	</body>
</html>

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