Are templates fully customizable or I can change only photos inside them? Since I started my own app from scratch, now I would like to use some templates Zappter is offering but i don't want to install them if i can't tweak some design in elements
Design is fully customizable.
The template is just your start design. From the Sidebar, you can expand the Design section and click on Design button.
From there you can create your unique design for your app.