That's not even possible in theory. If my app uses half system controls and half custom-drawn controls that I have painstakingly made to look like (I expected)

That's not even possible in theory. If my app uses half system controls and half custom-drawn controls that I have painstakingly made to look like (I expected) the system controls to look, then if you swap out the system controls now you made the app inconsistent internally, which is just worse. Windows has some consistency flaws, e.g. how they can't make their own settings have a consistent UI, but releasing more toolkits with different looks can't change how past ones look, it's just not possible. Having apps being consistent internally is the best you can hope for, and that obviously is the same for Linux/MacOS as well.