How to insert dark mode in iPhone 6

You're able to do this via an accessibility feature to invert colors & switch between light & dark mode performing a triple click of the home key. Go to settings> Accessibility> Accessability shortcut> Classic invert. I set this up on my iPhone 6s Plus many yrs ago because I have old age eyes & the black background is SO much easier on the eyes.

I just got an iphone 13 Pro Max & don't think the dark mode feature is as good as the feature I describe above so I turned off dark mode & set this same accessibility shortcut on my new phone. The triple click to change between light to dark modes is done via a triple click of the on/off button (on right side) since there's no home button on the newer phones.

Buy a newer phone. The iPhone 6 cannot support dark mode.