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

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

config/locales/en.yml

@@ -1769,6 +1769,7 @@ en:
     contrast: Mastodon (High contrast)
     default: Mastodon (Dark)
     mastodon-light: Mastodon (Light)
+    system: Automatic (use system theme)
   time:
     formats:
       default: "%b %d, %Y, %H:%M"

config/settings.yml

@@ -16,7 +16,7 @@ defaults: &defaults
   show_staff_badge: true
   preview_sensitive_media: false
   noindex: false
-  theme: 'default'
+  theme: 'system'
   trends: true
   trends_as_landing_page: true
   trendable_by_default: false