Support "system" theme setting (light/dark theme depending on user system preference) (#29748)

Co-authored-by: Nishiki Liu <hello@nshki.com>

app/lib/themes.rb

@@ -11,6 +11,6 @@ class Themes
   end
 
   def names
-    @conf.keys
+    ['system'] + @conf.keys
   end
 end