use custom main favicon

app/helpers/appearances_helper.rb

@@ -35,4 +35,8 @@ module AppearancesHelper
       render 'shared/logo_type.svg'
     end
   end
+
+  def brand_favicon
+    brand_item&.favicon
+  end
 end

app/helpers/page_layout_helper.rb

@@ -39,6 +39,7 @@ module PageLayoutHelper
   end
 
   def favicon
+    return brand_favicon.default.url if brand_favicon
     return 'favicon-yellow.ico' if Gitlab::Utils.to_boolean(ENV['CANARY'])
     return 'favicon-green.ico' if Rails.env.development?
 

spec/helpers/page_layout_helper_spec.rb

@@ -60,6 +60,11 @@ describe PageLayoutHelper do
       stub_env('CANARY', 'true')
       expect(helper.favicon).to eq 'favicon-yellow.ico'
     end
+
+    it 'uses the custom favicon if an favicon appearance is present' do
+      create :appearance, favicon: fixture_file_upload(Rails.root.join('spec/fixtures/dk.png'))
+      expect(helper.favicon).to match %r{/uploads/-/system/appearance/favicon/\d+/default_dk.ico}
+    end
   end
 
   describe 'page_image' do