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

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

app/views/layouts/application.html.haml

@@ -27,7 +27,7 @@
     %title= html_title
 
     = stylesheet_pack_tag 'common', media: 'all', crossorigin: 'anonymous'
-    = stylesheet_pack_tag current_theme, media: 'all', crossorigin: 'anonymous'
+    = theme_style_tags current_theme
     -# Needed for the wicg-inert polyfill. It needs to be on it's own <style> tag, with this `id`
     = stylesheet_pack_tag 'inert', media: 'all', id: 'inert-style'
 

app/views/layouts/embedded.html.haml

@@ -12,7 +12,7 @@
       %link{ rel: 'dns-prefetch', href: storage_host }/
 
     = stylesheet_pack_tag 'common', media: 'all', crossorigin: 'anonymous'
-    = stylesheet_pack_tag Setting.default_settings['theme'], media: 'all', crossorigin: 'anonymous'
+    = theme_style_tags Setting.theme # Use the admin-configured theme here, even if logged in
     = javascript_pack_tag 'common', integrity: true, crossorigin: 'anonymous'
     = preload_pack_asset "locale/#{I18n.locale}-json.js"
     = render_initial_state

app/views/layouts/error.html.haml

@@ -6,7 +6,7 @@
     %title= safe_join([yield(:page_title), Setting.default_settings['site_title']], ' - ')
     %meta{ content: 'width=device-width,initial-scale=1', name: 'viewport' }/
     = stylesheet_pack_tag 'common', media: 'all', crossorigin: 'anonymous'
-    = stylesheet_pack_tag Setting.default_settings['theme'], media: 'all', crossorigin: 'anonymous'
+    = theme_style_tags Setting.default_settings['theme']
     = javascript_pack_tag 'common', crossorigin: 'anonymous'
     = javascript_pack_tag 'error', crossorigin: 'anonymous'
   %body.error