Coverage improvement round-out following up previous work (#23987)

spec/controllers/custom_css_controller_spec.rb

@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+describe CustomCssController do
+  render_views
+
+  describe 'GET #show' do
+    it 'returns http success' do
+      get :show
+      expect(response).to have_http_status(200)
+    end
+  end
+end