The gray line that you are referring to is the app nav. You can hide the nav with the app-level css if you like, using the following:
You'll notice immediately that the background of the app does not expand to the entire height of the window. To fix this, you'll want to add:
calc( 100vh - 54px)
We plan to give you total control over the nav in your apps in the near future, but at the moment this css tweak is the best way to remove it entirely.
Note that if you add more than one template to your app, you wont be able to access them if you do this, because you'll be hiding your nav.