Dark mode All apps

tharaga ~ Dark Mode was introduced in iOS 13, but your iPhone 6 is not compatible with iOS 13:

iOS 13 and iPadOS are compatible with these devices

...So to have Dark Mode you must buy one of the newer iPhone models listed in the article above.

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Fyi only apps whose APIs are compliant with latest iOS rev. will get the setting.

Regarding how to apply it, please see HT210332