You have two sub page templates to choose from.  

1. The "Subpage Default" displays your page with a sidebar and a sidebar navigation.
2. The "Subpage Fullwidth" displays your page with no sidebar and full width sections.

Your template "Subpage Default" is your default page template and the "Style Guide" page has been setup to display your site default styles.

*NOTE - The index.php is your home template and assigned automatically to your home page.  This templates operates the same as your "Subpage Fullwidth" template.