Set "admin" body class from admin nested layout (#31269)

spec/helpers/application_helper_spec.rb

@@ -8,7 +8,16 @@ RSpec.describe ApplicationHelper do
       before { helper.extend controller_helpers }
 
       it 'uses the controller body classes in the result' do
-        expect(helper.body_classes).to match(/modal-layout compose-standalone/)
+        expect(helper.body_classes)
+          .to match(/modal-layout compose-standalone/)
+          .and match(/theme-default/)
+      end
+
+      it 'includes values set via content_for' do
+        helper.content_for(:body_classes) { 'admin' }
+
+        expect(helper.body_classes)
+          .to match(/admin/)
       end
 
       private