Add a site-wide dark mode

src/app.html

@@ -3,6 +3,19 @@
 	<head>
 		<meta charset="utf-8" />
 		<meta name="viewport" content="width=device-width, initial-scale=1" />
+		<script>
+			(function () {
+				try {
+					const theme = localStorage.getItem('theme');
+					const dark =
+						theme === 'dark' ||
+						(!theme && window.matchMedia('(prefers-color-scheme: dark)').matches);
+					document.documentElement.classList.toggle('dark', dark);
+				} catch (e) {
+					// ignore
+				}
+			})();
+		</script>
 		%sveltekit.head%
 	</head>