Add note about iOS 13 requirement

Author: brentvatne

docs/themes.md

@@ -25,6 +25,8 @@ This will take care of styling the stack navigator, bottom tab navigator, and dr
 
 At the time of writing, `react-native` does not currently support detecting the operating system color scheme preferences in the core ([you can follow this pull request](https://github.com/facebook/react-native/pull/26172)). Until it is part of core and you have updated to the version that includes it, you can use `react-native-appearance`.
 
+You will need iOS 13 (currently in beta) to actually be able to toggle dark mode through system settings.
+
 > Note: if you use the Expo managed workflow, this requires SDK 35+
 
 First, you need to install `react-native-appearance`. [Follow the instructions in the README](https://github.com/expo/react-native-appearance).

website/versioned_docs/version-3.x/themes.md

@@ -26,6 +26,8 @@ This will take care of styling the stack navigator, bottom tab navigator, and dr
 
 At the time of writing, `react-native` does not currently support detecting the operating system color scheme preferences in the core ([you can follow this pull request](https://github.com/facebook/react-native/pull/26172)). Until it is part of core and you have updated to the version that includes it, you can use `react-native-appearance`.
 
+You will need iOS 13 (currently in beta) to actually be able to toggle dark mode through system settings.
+
 > Note: if you use the Expo managed workflow, this requires SDK 35+
 
 First, you need to install `react-native-appearance`. [Follow the instructions in the README](https://github.com/expo/react-native-appearance).