I have been using iOS 13 on my new iPhone 11 for a while now. I really like it, especially the Dark Mode feature. However, the first few days I was using iOS 13 the iPhone was switching back to Light Mode every morning. I would then set it back to Dark Mode. I could not figure this out until I took the time to really look at the Appearance Settings.
Dark Mode Toggle
You can toggle Dark and Light Mode on your iOS device using the Display & Brightness Setting. You can also do it in the Control Center. But, if you want the iOS Device to remain in Dark Mode as a Default, then you have to fix a couple of Settings.
From your Home Screen go into the Settings area:
Once you are in Settings, tap on Display & Brightness:
You will be taken to this screen:
Here is the deal. When you have Automatic activated, iOS automatically adjusts your iPhone from Dark to Light mode based on the “Options”. Here is the way mine used to be:
Duh! It was set to go to Light Mode at Sunrise. The way to fix this if you wish to remain in either Light or Dark mode as a Default is to turn off Automatic:
When you turn off Automatic the iPhone will remain in whatever Appearance Mode you place it. In my case, that is Dark Mode most of the time.
Sometimes the operating systems released by Apple are just too smart for our own good. Anyway, adjusting Dark and Light Mode is pretty easy once you take the time to actually look at things.