Fix missing CSS variables in Darkmode

This MR fixes a bug introduced by:
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/64806

The CSS variables for Darkmode were not available in the page-bundle
anymore.

Changelog: fixed

app/views/layouts/_head.html.haml

@@ -32,6 +32,9 @@
   - if page_canonical_link
     %link{ rel: 'canonical', href: page_canonical_link }
 
+  - if user_application_theme == 'gl-dark'
+    = preload_link_tag(path_to_stylesheet('application_utilities_dark'))
+  - else
     = preload_link_tag(path_to_stylesheet('application_utilities'))
   = yield :prefetch_asset_tags
 
@@ -40,6 +43,8 @@
   = render 'layouts/startup_css', { startup_filename: local_assigns.fetch(:startup_filename, nil) }
   - if user_application_theme == 'gl-dark'
     = stylesheet_link_tag_defer "application_dark"
+    = yield :page_specific_styles
+    = stylesheet_link_tag_defer "application_utilities_dark"
   - else
     = stylesheet_link_tag_defer "application"
     = yield :page_specific_styles