A sitewide theming system is currently under development, however, you currently cannot change the theme or template of a page.
Nonetheless, you can customize the layout on each of your pages, to suit your needs and accommodate your apps.
In order to change the layout of a page, click on the shelf (control bar at the bottom) where it says “Layout” and select one from the available. Notice the disclaimer:
* Layout changes will ONLY apply to this page
Update! You can further customize the layout by adding and removing blocks. The Layout Builder has just been released!
My “shelf” doesnt include the Layout option. It has everything else except that. Frustrating.