Apple does allow you to chose between dark and light mode, as well as accent- and highlight colours.
Beyond that, you quickly get to a point where whole layouts would have to change. Doing so, across three to five platforms, would be a major undertaking for Apple itself. And app developers would never be able to consistently support multiple design paradigms, leading to a fractured experience.
Beyond that, you quickly get to a point where whole layouts would have to change. Doing so, across three to five platforms, would be a major undertaking for Apple itself. And app developers would never be able to consistently support multiple design paradigms, leading to a fractured experience.