Convert admin/settings controller specs to system specs (#31548)

spec/system/admin/settings/appearance_spec.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+describe 'Admin::Settings::Appearance' do
+  it 'Saves changes to appearance settings' do
+    sign_in admin_user
+    visit admin_settings_appearance_path
+
+    fill_in custom_css_field,
+            with: 'html { display: inline; }'
+
+    click_on submit_button
+
+    expect(page)
+      .to have_content(success_message)
+  end
+
+  def custom_css_field
+    form_label 'form_admin_settings.custom_css'
+  end
+end