Regular developers might have some problem with this, if they've built their apps with nib-files or storyboards. Doing it programmatically however, means changing a few values.
It's like when the bigger phones were released, if you had a lot of hard coded view sizes (values that don't seem to originate from anything, "magic numbers") - you were in for a lot of work. Experienced programmers never use magic numbers for anything. They know circumstances will always change and, in addition, avoiding magic numbers gives you cleaner, more readable code.
Now we are in a similar situation again. Are your views hard coded to be bright/light? Or have you been using consistent variables for your colors all through your app? Now that decision will pay off.
Of course, to conclude this touting-my-own-horn-post, if you are using something like AutoStandards, darkness comes for free.