Fix system theme being included twice (#37526)

app/helpers/theme_helper.rb

@@ -18,14 +18,10 @@ module ThemeHelper
   end
 
   def theme_style_tags(theme)
-    if theme == 'system'
-      ''.html_safe.tap do |tags|
-        tags << vite_stylesheet_tag('themes/mastodon-light', type: :virtual, media: 'not all and (prefers-color-scheme: dark)', crossorigin: 'anonymous')
-        tags << vite_stylesheet_tag('themes/default', type: :virtual, media: '(prefers-color-scheme: dark)', crossorigin: 'anonymous')
-      end
-    else
-      vite_stylesheet_tag "themes/#{theme}", type: :virtual, media: 'all', crossorigin: 'anonymous'
-    end
+    # TODO: get rid of that when we retire the themes and perform the settings migration
+    theme = 'default' if %w(mastodon-light contrast system).include?(theme)
+
+    vite_stylesheet_tag "themes/#{theme}", type: :virtual, media: 'all', crossorigin: 'anonymous'
   end
 
   def theme_color_tags(color_scheme)

spec/helpers/theme_helper_spec.rb

@@ -9,17 +9,10 @@ RSpec.describe ThemeHelper do
     context 'when using "system" theme' do
       let(:theme) { 'system' }
 
-      it 'returns the mastodon-light and application stylesheets with correct color schemes' do
+      it 'returns the default theme' do
         expect(html_links.first.attributes.symbolize_keys)
           .to include(
-            # This is now identical to the default theme & will be unified very soon
-            href: have_attributes(value: match(/default/)),
-            media: have_attributes(value: 'not all and (prefers-color-scheme: dark)')
-          )
-        expect(html_links.last.attributes.symbolize_keys)
-          .to include(
-            href: have_attributes(value: match(/default/)),
-            media: have_attributes(value: '(prefers-color-scheme: dark)')
+            href: have_attributes(value: match(/default/))
           )
       end
     end
@@ -41,8 +34,7 @@ RSpec.describe ThemeHelper do
       it 'returns the theme stylesheet without color scheme information' do
         expect(html_links.first.attributes.symbolize_keys)
           .to include(
-            href: have_attributes(value: match(/default/)),
-            media: have_attributes(value: 'all')
+            href: have_attributes(value: match(/default/))
           )
       end
     end