There are some issues with colours on scratchbox, it should look better on a device. E.g. the turquoise you see, is actually lime green...

Closing dialogs by tapping outside their area (which is dimmed) provides by far the biggest possible click target. Putting a visible X somewhere would only distract from that, and it takes just a few seconds to learn, then it feels completely natural. I bet you also learned that you can close menus on your desktop by clicking somewhere else.

lcuk: The X in the background is getting dimmed like the rest of the inactive background, and tapping anywhere on that space will only close the dialog of course.