Fix custom css cache miss (#33583)

app/helpers/theme_helper.rb

@@ -45,7 +45,9 @@ module ThemeHelper
   end
 
   def cached_custom_css_digest
-    Rails.cache.read(:setting_digest_custom_css)
+    Rails.cache.fetch(:setting_digest_custom_css) do
+      Setting.custom_css&.then { |content| Digest::SHA256.hexdigest(content) }
+    end
   end
 
   def theme_color_for(theme)

spec/helpers/theme_helper_spec.rb

@@ -80,18 +80,37 @@ RSpec.describe ThemeHelper do
   end
 
   describe '#custom_stylesheet' do
+    let(:custom_css) { 'body {}' }
+    let(:custom_digest) { Digest::SHA256.hexdigest(custom_css) }
+
+    before do
+      Setting.custom_css = custom_css
+    end
+
     context 'when custom css setting value digest is present' do
-      before { Rails.cache.write(:setting_digest_custom_css, '1a2s3d4f1a2s3d4f') }
+      before { Rails.cache.write(:setting_digest_custom_css, custom_digest) }
 
       it 'returns value from settings' do
         expect(custom_stylesheet)
-          .to match('/css/custom-1a2s3d4f.css')
+          .to match("/css/custom-#{custom_digest[...8]}.css")
       end
     end
 
-    context 'when custom css setting value digest is not present' do
+    context 'when custom css setting value digest is expired' do
       before { Rails.cache.delete(:setting_digest_custom_css) }
 
+      it 'returns value from settings' do
+        expect(custom_stylesheet)
+          .to match("/css/custom-#{custom_digest[...8]}.css")
+      end
+    end
+
+    context 'when custom css setting is not present' do
+      before do
+        Setting.custom_css = nil
+        Rails.cache.delete(:setting_digest_custom_css)
+      end
+
       it 'returns default value' do
         expect(custom_stylesheet)
           .to be_blank