How do you set or style default "New Page" in Upfront Editor (from theme created in builde

Scenario:
- I've created a theme called "New Start" in Upfront Builder.
- I exported and installed the New Start theme in a separate domain (newstart.iconmarketing.works) .
- Now managing the site in Upfront Editor (separate hosting/worpress account), creating a "New Page" gives me a generic default layout as seen below:

How do I edit or set a template in Upfront Builder so that when I create a "New Page" template in Editor looks more like this?:

Also...What's the best practice for updating a theme after the following scenario?
- Theme created in Upfront Builder
-Theme exported and installed on a separate web site/hosting
-Changes/updates made to original theme in Upfront Builder and now need changes to reflect on external site.

Do I export the theme and delete the active theme on the external site and reinstall? Or is there a better way?

Hope this makes sense.

-Leno