Assuming you are referring to a map-based login screen, the black part around it is the part of the map control that isn't being occupied by your view. You'll need to increase your view size to get rid of that space, which means you will probably need to increase the size of your image. All the better reason to study up on interfaces!
If you have black space around your viewing area, your view isn't filling the map. It won't fill the map for everyone very easily, either, because people have different screen sizes. As for your question about demos, I'm assuming you mean demos on interfaces?
Plenty of libraries and demos use interfaces, especially Forum_account's. You should probably check out the skin reference (Open up Dream Maker and go under the Help menu and click Skin Reference). Make a new interface file and poke around a bit. See what things do. Search the forums, too, as they hold a lot of answered questions about interfaces.
