How on earth is the same set of UI icons supposed to keep their readability against vastly different background colors? If you want to do a proper job of that,

How on earth is the same set of UI icons supposed to keep their readability against vastly different background colors? If you want to do a proper job of that, your application needs to become aware of dark mode anyway and ship with a separate set of icons. Yes, in terms of raw flexibility exposed to the user, customizable color schemes look great on the surface. They work to some degree, but fail utterly for a wide range of cases in practice. Not having them is progress after a fashion because it means giving up on an flawed idea. I will admit, though, that it is not easy to see why it is flawed when you haven't tried to design custom controls that have to convey more state than can be expressed using the standard OS colir palette.