For low vision users contrast is a accessibility necessity. Converting to a dark background with white text, instead of the usual black on white makes a huge difference. Literally the difference between being able to see to use an app or not.
So much so, that I used to root Android phones and apply dark themed apps. This would allow me to configure apps such as Facebook to have a black background with white text. Therefore, hugely increasing the contrast thus the usability.
With WWDC around the corner there is a wonderful Apple rumour about “Dark Mode”. A Dark Mode for the standard iOS apps would be fantastic, Mail for example would be incredible with a Dar Mode. The interesting point in the rumour however, is that of a Dark Mode API.
Allowing 3rd party apps to easily include a dark mode could be a massive game changer for low vision users. I can only hope this rumour sees fruition and that it is a permanent setting within the accessibility settings.