It has been suggested to ship some dark themes in gnome-themes in the past, but I would go a step further and propose using a dark(ish) theme by default in stock GNOME. Having a dark theme would make perfect sense for vanilla GNOME. It would expose theming issues which are easily ignored when all you run is a light background. It will not really affect huge number of end users as most distros ship their custom themes though. But the core developers will have it up their faces
No other desktop I know is using dark widgets, it would make up for a distinct GNOME visual style to an extent the nature-theme we discussed at Guadec ever would. Let the distros be held down by conservatism, let’s be bold and go dark!