Dark mode in iOS 12 is wishful thinking. As we have seen with macOS, doing dark mode effectively requires an API so 3rd party applications can tap into the system-wide setting as well. Otherwise, only 1st party apps will conform to the dark mode while others will still use a light theme. If Apple were to suddenly announce an iOS dark mode with the release of iOS 12, literally no 3rd party apps will support it at launch since developers will have had no idea it was coming. It would also probably be buggy since it would not have gone through any beta testing. The result would be annoyed users AND annoyed developers, which is not a situation I think Apple wants to get itself into.
As others have said, I don’t think a dark mode is coming before iOS 13 (if it ever gets added at all). It seems like a no-brainer given the potential battery savings that could be had on OLED screens. But, as usual, Apple knows best...🙄