New: Auto theme option to match OS theme

Closes #3165

Co-authored-by: Qstick <qstick@gmail.com>
(cherry picked from commit 4ca5a213fa0fc29ed93e7e31b080728d6fa7f1f3)

frontend/src/Styles/Themes/index.js

@@ -1,7 +1,11 @@
 import * as dark from './dark';
 import * as light from './light';
 
+const defaultDark = window.matchMedia('(prefers-color-scheme: dark)').matches;
+const auto = defaultDark ? { ...dark } : { ...light };
+
 export default {
+  auto,
   light,
   dark
 };