OS X Yosemite and El Capitan's implementation of Dark Mode applies only to the menu bar, Dock and Spotlight. It does not affect any apps. Well if you want the same implementation in iOS since OS X has done it, only the dock will be dark…
Dark Mode in iOS arising from iOS tweaks are terribly hacky and don't work in certain apps, resulting in crashes and unusable apps, etc. If iOS were to implement it, only compatible apps (which will take up another long cycle of developer updates) will go dark. I doubt you want to enter an app that's dark and when you switch to another app it goes completely white.
There really isn't much motivation for Apple to implement Dark Mode in iOS right now. First of all, there are brightness controls and a Reduce White Point option in Accessibility. Second of all Dark Mode is not going to bring any additional battery life to the iPhone/iPad since the screens are LCD displays. Until iPhones/iPads are released with OLED screens it is unlikely iOS will ever have dark mode.
Under iOS 9 in Safari, you can upload files using iCloud Drive. In Mail you can also save attachments to iCloud Drive.
Since previous iOS versions you can do an iCloud backup and select what apps have their app data backed up. From your device, go to look for your iCloud storage usage, select your backups and choose the apps you want backed up.