accept `/?__theme=dark`

src/routes/+layout.svelte

@@ -1,5 +1,11 @@
 <script>
 	import "../app.css";
+	import { browser } from '$app/environment';
+	import { page } from '$app/stores';
+
+	$: if (browser && $page.url.searchParams.get('__theme') === 'dark') {
+		document.documentElement.classList.add('dark');
+	}
 </script>
 
 <slot></slot>