Maybe not now, but back in the day, that was utter BS. You pretty mucch had to go out of your way not to have your app use whatever colour palette the user had decided on. Well, on Windows, at least. (And here I thought people used to brag about programming tools on Linux being better than on Windows; are you saying that in at least this respect, they werern't better even then?)
> Only trivial apps would work by changing some system-wide palette.
That must be why almost ALL Windows apps up until at least ~2010 worked just fine by changing the system-wide palette.
> Also, if there is any artwork (button icons etc) then that has to be done in two or more sets, for varying backgrounds. Few icons would work well on both a light and dark background.
Oh, it worked well enough. (Mostly, I suppose, by the super-advanced trick of using colours pretty much midway between light and dark.)