Fallback to default theme when admin-selected theme does not exist (#38703)

app/helpers/theme_helper.rb

@@ -47,9 +47,15 @@ module ThemeHelper
   end
 
   def current_theme
-    return Setting.theme unless Themes.instance.names.include? current_user&.setting_theme
+    available_themes = Themes.instance.names
 
-    current_user.setting_theme
+    user_theme = current_user&.setting_theme
+    return user_theme if user_theme && available_themes.include?(user_theme)
+
+    site_theme = Setting.theme
+    return site_theme if available_themes.include?(site_theme)
+
+    'default' # Fallback
   end
 
   def color_scheme

spec/helpers/theme_helper_spec.rb

@@ -109,10 +109,19 @@ RSpec.describe ThemeHelper do
       end
 
       context 'when theme is changed in settings' do
-        before { Setting.theme = 'contrast' }
+        before do
+          allow(Themes.instance).to receive(:names).and_return(%w(default contrast))
+          Setting.theme = 'contrast'
+        end
 
         it { is_expected.to eq('contrast') }
       end
+
+      context 'when theme is changed to invalid value' do
+        before { Setting.theme = 'fakethemename' }
+
+        it { is_expected.to eq('default') }
+      end
     end
 
     context 'when user is signed in' do