respect system theme, without FOUC

apps/svelte.dev/src/app.html

@@ -19,11 +19,6 @@
 		/>
 
 		<script>
-			document.documentElement.classList.add(
-				JSON.parse(localStorage.getItem('svelte:theme'))?.current ??
-					(window.matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light')
-			);
-
 			document.documentElement.classList.add(
 				`font-${localStorage.getItem('svelte:font') ?? 'elegant'}`
 			);

apps/svelte.dev/src/routes/(authed)/playground/[id]/+page.svelte

@@ -5,7 +5,7 @@
 	import { afterNavigate, goto, replaceState } from '$app/navigation';
 	import type { Gist } from '$lib/db/types';
 	import { Repl } from '@sveltejs/repl';
-	import { theme } from '@sveltejs/site-kit/stores';
+	import { theme } from '@sveltejs/site-kit/state';
 	import { mapbox_setup } from '../../../../config.js';
 	import AppControls from './AppControls.svelte';
 	import { compress_and_encode_text, decode_and_decompress_text } from './gzip.js';
@@ -240,7 +240,7 @@
 				injectedJS={mapbox_setup}
 				{onchange}
 				{download}
-				previewTheme={$theme.current}
+				previewTheme={theme.current}
 			/>
 		</div>
 	{/if}

apps/svelte.dev/src/routes/(authed)/playground/[id]/embed/+page.svelte

@@ -1,7 +1,7 @@
 <script lang="ts">
 	import { browser } from '$app/environment';
 	import { afterNavigate, replaceState } from '$app/navigation';
-	import { theme } from '@sveltejs/site-kit/stores';
+	import { theme } from '@sveltejs/site-kit/state';
 	import { Repl } from '@sveltejs/repl';
 	import { mapbox_setup } from '../../../../../config.js';
 	import { page } from '$app/state';
@@ -50,7 +50,7 @@
 			{relaxed}
 			can_escape
 			injectedJS={mapbox_setup}
-			previewTheme={$theme.current}
+			previewTheme={theme.current}
 			embedded
 		/>
 	{/if}

apps/svelte.dev/src/routes/tutorial/[...slug]/Output.svelte

@@ -5,7 +5,7 @@
 <script lang="ts">
 	import { browser, dev } from '$app/environment';
 	import { afterNavigate } from '$app/navigation';
-	import { theme, type Theme } from '@sveltejs/site-kit/stores';
+	import { theme } from '@sveltejs/site-kit/state';
 	import { onMount } from 'svelte';
 	import Chrome from './Chrome.svelte';
 	import Loading from './Loading.svelte';
@@ -43,18 +43,6 @@
 		clearTimeout(timeout);
 	});
 
-	function change_theme(theme: Theme) {
-		if (!iframe) return;
-
-		try {
-			const url = new URL(iframe.src);
-
-			url.searchParams.set('theme', theme.current);
-
-			iframe.src = url.href;
-		} catch {}
-	}
-
 	let timeout: any;
 
 	async function handle_message(e: MessageEvent) {
@@ -97,7 +85,7 @@
 		parentNode?.removeChild(iframe);
 
 		const url = new URL(src);
-		url.searchParams.set('theme', $theme.current);
+		url.searchParams.set('theme', theme.current);
 
 		iframe.src = url.href;
 		parentNode?.appendChild(iframe);
@@ -112,8 +100,17 @@
 	$effect(() => {
 		if (adapter_state.base) set_iframe_src(adapter_state.base + (path = exercise.path));
 	});
+
 	$effect(() => {
-		change_theme($theme);
+		if (!iframe) return;
+
+		try {
+			const url = new URL(iframe.src);
+
+			url.searchParams.set('theme', theme.current);
+
+			iframe.src = url.href;
+		} catch {}
 	});
 </script>
 

apps/svelte.dev/src/routes/tutorial/[...slug]/OutputRollup.svelte

@@ -4,7 +4,7 @@
 	import Viewer from '@sveltejs/repl/viewer';
 	// @ts-expect-error TODO types
 	import Console, { type Log } from '@sveltejs/repl/console';
-	import { theme } from '@sveltejs/site-kit/stores';
+	import { theme } from '@sveltejs/site-kit/state';
 	import Chrome from './Chrome.svelte';
 	import Loading from './Loading.svelte';
 	import { adapter_state, update } from './adapter.svelte';
@@ -37,7 +37,7 @@
 			can_escape
 			onLog={(l: Log[]) => (logs = l)}
 			{bundle}
-			theme={$theme.current}
+			theme={theme.current}
 			injectedCSS="@import '/tutorial/shared.css';"
 		/>
 	{/if}

packages/repl/src/lib/Input/RunesInfo.svelte

@@ -5,7 +5,7 @@
 	let { runes }: { runes: boolean } = $props();
 
 	const { workspace, svelteVersion } = get_repl_context();
-	const majorVersion = Number(svelteVersion.split(".")[0]);
+	const majorVersion = Number(svelteVersion.split('.')[0]);
 </script>
 
 <Dropdown align="right">
@@ -19,8 +19,8 @@
 		<div class="popup">
 			{#if Number.isInteger(majorVersion) && majorVersion < 5}
 				<p>
-					<a href="/blog/runes">Runes</a> are available from Svelte 5 onwards, and this playground is
-					using Svelte {svelteVersion}.
+					<a href="/blog/runes">Runes</a> are available from Svelte 5 onwards, and this playground
+					is using Svelte {svelteVersion}.
 				</p>
 			{:else if workspace.current.name.endsWith('.svelte.js')}
 				<p>

packages/site-kit/package.json

@@ -73,6 +73,9 @@
 		"./actions": {
 			"default": "./src/lib/actions/index.ts"
 		},
+		"./state": {
+			"default": "./src/lib/state/index.ts"
+		},
 		"./stores": {
 			"default": "./src/lib/stores/index.ts"
 		},

packages/site-kit/src/lib/components/ThemeToggle.svelte

@@ -1,31 +1,37 @@
 <script lang="ts">
-	import { on } from 'svelte/events';
-	import { theme } from '../stores';
+	import { theme } from '../state';
 
 	function toggle() {
-		const next = $theme.current === 'light' ? 'dark' : 'light';
-		const system = window.matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light';
-
-		$theme.preference = next === system ? 'system' : next;
-		$theme.current = next;
+		theme.current = theme.current === 'light' ? 'dark' : 'light';
 	}
 
 	$effect(() => {
-		if ($theme.preference === 'system') {
-			const query = window.matchMedia('(prefers-color-scheme: dark)');
-
-			return on(query, 'change', (e) => {
-				$theme.current = e.matches ? 'dark' : 'light';
-			});
-		}
+		document.documentElement.classList.remove('light', 'dark');
+		document.documentElement.classList.add(theme.current);
 	});
 </script>
 
+<svelte:head>
+	<script>
+		{
+			const theme = localStorage.getItem('sv:theme');
+
+			document.documentElement.classList.add(
+				theme === 'system'
+					? window.matchMedia('(prefers-color-scheme: dark)').matches
+						? 'dark'
+						: 'light'
+					: theme
+			);
+		}
+	</script>
+</svelte:head>
+
 <button
 	onclick={toggle}
 	class="raised icon"
 	type="button"
-	aria-pressed={$theme.current === 'dark'}
+	aria-pressed={theme.current === 'dark'}
 	aria-label="Toggle dark mode"
 ></button>
 

packages/site-kit/src/lib/state/Persisted.svelte.ts

@@ -0,0 +1,39 @@
+import { on } from 'svelte/events';
+import { createSubscriber } from 'svelte/reactivity';
+
+export class Persisted<T extends string = string> {
+	#key: string;
+	#storage: Storage | undefined;
+	#fallback: T;
+	#version = $state(0);
+
+	#subscribe = createSubscriber((update) => {
+		return on(window, 'storage', (e) => {
+			if (e.key === this.#key) {
+				update();
+			}
+		});
+	});
+
+	constructor(
+		key: string,
+		fallback: T,
+		storage = typeof localStorage === 'undefined' ? undefined : localStorage
+	) {
+		this.#key = key;
+		this.#fallback = fallback;
+		this.#storage = storage;
+	}
+
+	get current() {
+		this.#subscribe(); // handle cross-tab updates
+		this.#version; // handle same-tab updates
+
+		return (this.#storage?.getItem(this.#key) as T) ?? this.#fallback;
+	}
+
+	set current(v: T) {
+		this.#storage?.setItem(this.#key, v);
+		this.#version += 1;
+	}
+}

packages/site-kit/src/lib/state/index.ts

@@ -0,0 +1,2 @@
+export { Persisted } from './Persisted.svelte';
+export { theme } from './theme.svelte';

packages/site-kit/src/lib/state/theme.svelte.ts

@@ -0,0 +1,18 @@
+import { MediaQuery } from 'svelte/reactivity';
+import { Persisted } from './Persisted.svelte';
+
+class Theme {
+	#preference = new Persisted<'system' | 'light' | 'dark'>('sv:theme', 'system');
+	#query = new MediaQuery('prefers-color-scheme: dark');
+	#system = $derived<'dark' | 'light'>(this.#query.current ? 'dark' : 'light');
+
+	get current() {
+		return this.#preference.current === 'system' ? this.#system : this.#preference.current;
+	}
+
+	set current(value: 'light' | 'dark') {
+		this.#preference.current = value === this.#system ? 'system' : value;
+	}
+}
+
+export const theme = new Theme();

packages/site-kit/src/lib/stores/index.ts

@@ -2,4 +2,3 @@ export { mql } from './mql';
 export { nav_open, on_this_page_open, overlay_open, should_nav_autohide } from './nav';
 export { reduced_motion } from './reduced-motion';
 export { search_query, search_recent, searching } from './search';
-export { theme, type Theme } from './theme';