By clicking on the Design, there you can setup your app design. You can setup the design of the whole app, so you can make changes into:
General - here you can change the color of the app, add a background image, button color, button text and etc.
Navigation bar - set up the design of the navigation bar like background color, border color and etc.
List design - set up the list design
Side menu colors - choose the color for your side menu