How to set rounded corners on screen body like Pinterest?

I would like to create rounded corners on the outer body of the app screen.

The hope is to achieve something inspired by the current Pinterest app (2015-03-23). Ideally this would be rounded corners in the buttom left/right as well as the top. Screenshot attached below

I tried sass with body{@include border-radius(3px);} with no luck.

Does anyone have any ideas on how I might implement this?