A programmer has found interesting clues about Dark Mode and Mobile / Cellular Data toggle while exploring the iOS 10 code. Three images of iOS 10 Settings app donning the Dark theme were posted on Twitter. These latest leaks confirm that the Dark Mode feature is at least being considered and is under internal testing. So, iOS users may hope to get this feature in a future Beta release. System-wide Dark Theme is a desired feature surprisingly only available on Windows 10 Mobile out of the three major platforms. Google brought this feature in Android N Dev Preview 2 only to remove it in DP3.
Coming to Cellular / Mobile Data Toggle, it seems there is a blank button near the top of the control center. This is now speculated to be the Mobile Data toggle.