Add color-scheme meta tag (#8044)

Specifying `color-scheme` will theme the browser native elements that would otherwise not be automatically themed, such as scrollbars. `"dark light"` is a special value that respects the operating system preferences.

Author: mayank99

sites/kit.svelte.dev/src/app.html

@@ -4,6 +4,7 @@
 		<meta charset="utf-8" />
 		<meta name="viewport" content="width=device-width" />
 		<meta name="theme-color" content="#ff3e00" />
+		<meta name="color-scheme" content="dark light" />
 
 		<link rel="manifest" href="/manifest.json" />
 		<link rel="icon" type="image/png" href="/favicon.png" />