From the iOS 13 feature preview page:
Have Dark Mode turn on and off at a certain time or based on sunrise and sunset, which is great when you’re outside at night or using your iPhone before you go to bed.
I’d love to see iOS also alternate between dark and light mode depending on the available light (as determined by the ambient light sensor). I often find myself in a dark environment during the day, when the sun is shining full blast, yet I want Dark Mode to activate in those types of scenarios. Tweetbot has it. I hope iOS 13 gets it.