Even though most pages are editable through the template Builder, you have an option to use predefined custom templates for your pages. This is exceptionally usable, when you are building a separate page, which design is very different from your theme or a page that is stripped from any header/footer scripts, except your own.
Instructions:
Open your page through WordPress admin: Pages/Pages/your_page.
Select your ‘Custom Template’ as your page template of choice: