FEATURE: Enable system Horizon (#33374)

Enable Horizon theme for everyone.

config/site_settings.yml

@@ -4045,7 +4045,7 @@ experimental:
     default: false
   experimental_system_themes:
     type: list
-    default: ""
+    default: "horizon"
     hidden: true
     allow_any: false
     choices:

db/migrate/20250627022651_enable_horizon_for_existing_sites.rb

@@ -0,0 +1,10 @@
+# frozen_string_literal: true
+class EnableHorizonForExistingSites < ActiveRecord::Migration[7.2]
+  def change
+    execute <<~SQL if Migration::Helpers.existing_site?
+        INSERT INTO site_settings(name, data_type, value, created_at, updated_at)
+        VALUES('experimental_system_themes', 8, 'horizon', NOW(), NOW())
+        ON CONFLICT (name) DO NOTHING
+      SQL
+  end
+end

spec/requests/admin/color_schemes_controller_spec.rb

@@ -33,6 +33,7 @@ RSpec.describe Admin::ColorSchemesController do
       end
 
       it "filters colors belonging to experimental system themes" do
+        SiteSetting.experimental_system_themes = ""
         get "/admin/color_schemes.json"
         expect(response.status).to eq(200)
         scheme_names = response.parsed_body.map { |scheme| scheme["name"] }

spec/requests/admin/themes_controller_spec.rb

@@ -478,6 +478,7 @@ RSpec.describe Admin::ThemesController do
       end
 
       it "filters experimental system themes" do
+        SiteSetting.experimental_system_themes = ""
         get "/admin/themes.json"
         expect(response.status).to eq(200)
         theme_names = response.parsed_body["themes"].map { |theme| theme[:name] }