Background as an outline
How to use it
Cut out the outline of an element, letting the background show through as the outline.
When to use it
If you've got an interesting background that you want to show off, this is an easy way to bring it to the foreground, and at the same time not have to add any more elements on screen.
What to watch out for
If your background isn't distinctive enough, this technique might be very interesting. If your background is flat blue, so will your element outline be, and the user might not realise that you're letting the background through. In the example below, I applied a gradient to the background so you could tell that the background and the outline are the same.