Small refactor after last code review.

app/helpers/preferences_helper.rb

@@ -41,7 +41,7 @@ module PreferencesHelper
   end
 
   def user_application_theme
-    Gitlab::Themes.for_user(current_user).css_class
+    @user_application_theme ||= Gitlab::Themes.for_user(current_user).css_class
   end
 
   def user_color_scheme

lib/gitlab/themes.rb

@@ -73,13 +73,11 @@ module Gitlab
     private
 
     def default_id
+      @default_id ||= begin
         id = Gitlab.config.gitlab.default_theme.to_i
+        theme_ids = THEMES.map(&:id)
 
-      # Prevent an invalid configuration setting from causing an infinite loop
-      if id < THEMES.first.id || id > THEMES.last.id
-        APPLICATION_DEFAULT
-      else
-        id
+        theme_ids.include?(id) ? id : APPLICATION_DEFAULT
       end
     end
   end

spec/helpers/preferences_helper_spec.rb

 require 'spec_helper'
 
 describe PreferencesHelper do
-  describe 'dashboard_choices' do
+  describe '#dashboard_choices' do
     it 'raises an exception when defined choices may be missing' do
       expect(User).to receive(:dashboards).and_return(foo: 'foo')
       expect { helper.dashboard_choices }.to raise_error(RuntimeError)
@@ -26,7 +26,7 @@ describe PreferencesHelper do
     end
   end
 
-  describe 'user_application_theme' do
+  describe '#user_application_theme' do
     context 'with a user' do
       it "returns user's theme's css_class" do
         stub_user(theme_id: 3)
@@ -52,7 +52,7 @@ describe PreferencesHelper do
     end
   end
 
-  describe 'user_color_scheme' do
+  describe '#user_color_scheme' do
     context 'with a user' do
       it "returns user's scheme's css_class" do
         allow(helper).to receive(:current_user)