Extend custom CSS cache time with digest paths (#33207)

spec/routing/custom_css_routing_spec.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe 'Custom CSS routes' do
+  describe 'the legacy route' do
+    it 'routes to correct place' do
+      expect(get('/custom.css'))
+        .to route_to('custom_css#show')
+    end
+  end
+
+  describe 'the custom digest route' do
+    it 'routes to correct place' do
+      expect(get('/css/custom-1a2s3d4f.css'))
+        .to route_to('custom_css#show', id: 'custom-1a2s3d4f', format: 'css')
+    end
+  end
+end