Add Darkmode to sveltekit

hooks.server.ts

import type { Handle } from '@sveltejs/kit';
import { sequence } from '@sveltejs/kit/hooks';

const darkMode: Handle = async ({ event, resolve }) => {
	const darkModeCookie = event.cookies.get('darkmode') === 'true';
	const response = await resolve(event, {
		transformPageChunk: ({ html }) => html.replace('%darkmode%', darkModeCookie ? 'dark' : '')
	});

	return response;
};

export const handle = sequence(darkMode);

app.html

<body class="%darkmode%"></body>

If you are using tailwind edit the following config

app.css

@custom-variant dark (&:where(.dark, .dark *));

ToggleDarkmode.svelte

<script>
	import { onMount } from 'svelte';

	let isDarkMode = $state(false);

	function toggleDarkMode() {
		isDarkMode = !isDarkMode;
		document.cookie = `darkmode=${isDarkMode};path=/`;
		document.body.classList.toggle('dark', isDarkMode);
	}

	onMount(() => {
		isDarkMode = document.cookie.includes('darkmode=true');
		document.body.classList.toggle('dark', isDarkMode);
	});
</script>

<button onclick={toggleDarkMode}>
	{isDarkMode ? '🌙' : '☀️'}
</button>

Now the body element is gonna have the dark class once the cookie is set.