Now you're in User Registration here's where you choose the various ways in which a user can register themselves, and can customize the different registration walls that one might see through your app...
On the left, choose if you'd like to include Facebook, Twitter and/or Email as registration options. Continue choosing the Main Background Color, Header Text Color, Button Text Color, Button Color, and Text Links Color. It would be wise to continue using the same color aesthetic you've chosen throughout the App Design process to have a cohesive look and feel to your app.
Then bring your attention to the center, here you can change the view so you can see what the registration wall looks like at different points of a users experience - at First Save, general Registration Wall, New Subscriber, and what Email Registration and Email Login look like.
On the right, you can change the text and see how it looks on the various screens.
*NOTE* ONCE YOUR APP IS LIVE, ANY CHANGES YOU MAKE HERE AND SAVE WILL AUTOMATICALLY UPDATE IN THE APP.