Switching between the Light and Dark modes on iPhones should be easy. You can toggle between the modes simply by opening Settings > Display & Brightness and in the Appearance section and tapping on either the default Light or on Dark. But you can encounter a problem when you try to switch back to Light mode from Dark mode, when iPhone won’t let you switch to Light mode. Selecting the Light option in the phone’s Settings (or via the Light / Dark switch in iPhone’s Control Center) doesn’t do anything in that case. And, there’s no any kind of warning to let you know what’s the problem.
Here’s the solution that resolved the problem I encountered on my iPhone with iOS 13(.2.3):
Turn off Smart Invert.
You can’t switch from Dark to Light mode if the Smart Invert option on your iPhone is enabled. Smart Invert option is located in Settings > Accessibility > Display & Text Size. Turn Smart Invert off. Then go to Settings > Display & Brightness and tap on the Light option. You should be now in Light mode.
Note: This “how to” tip is applicable to iOS 13 or later compatible iPhones with iOS 13 or later installed. Compatible are iPhone 11, 11 Pro, 11 Pro Max, XS, XS Max, XR, X, iPhone 8, 8 Plus, 7, 7 Plus, iPhone SE, iPhone 6s, and 6s Plus.