Earlier quoted context omitted.
Supporting themes imposes an ongoing technical burden across the whole ecosystem. It's not worth it. You don't hear people say "I hate macs because I can't make my title bars purple".
If not for scrollbars color, I certainly dislike the macOS GUI for its straightjacket approach. (Also, poor window management.) I wish Gnome folks did not keep on copying it more and more. I see why it's easier to do, but KDE somehow manages. (And if one thinks that Apple UX design can't go wrong, remember Macbooks from a couple of years ago.)
macOS still has several traditional desktop affordances that are eschewed by GNOME, like full menus (not just hamburger junk drawer menus) and customizable toolbars.