Memoize non-existent custom appearances

This saves about 5 SQL calls per page if no custom appearance is
specified.

app/helpers/appearances_helper.rb

@@ -2,6 +2,7 @@
 
 module AppearancesHelper
   include MarkupHelper
+  include Gitlab::Utils::StrongMemoize
 
   def brand_title
     current_appearance&.title.presence || default_brand_title
@@ -25,7 +26,9 @@ module AppearancesHelper
   end
 
   def current_appearance
-    @appearance ||= Appearance.current
+    strong_memoize(:current_appearance) do
+      Appearance.current
+    end
   end
 
   def brand_header_logo

changelogs/unreleased/sh-strong-memoize-appearances.yml

+---
+title: Memoize non-existent custom appearances
+merge_request: 29957
+author:
+type: performance

spec/helpers/appearances_helper_spec.rb

@@ -8,6 +8,22 @@ describe AppearancesHelper do
     allow(helper).to receive(:current_user).and_return(user)
   end
 
+  describe '.current_appearance' do
+    it 'memoizes empty appearance' do
+      expect(Appearance).to receive(:current).once
+
+      2.times { helper.current_appearance }
+    end
+
+    it 'memoizes custom appearance' do
+      create(:appearance)
+
+      expect(Appearance).to receive(:current).once.and_call_original
+
+      2.times { helper.current_appearance }
+    end
+  end
+
   describe '#header_message' do
     it 'returns nil when header message field is not set' do
       create(:appearance)