You can find versions for both "toolkits" even in custom themes. For the end user it's mostly a non issue. Even with different backends it looks more consistent than Windows.
That's true, but the Iced theme that they're showing in the images looks like it would be very difficult to recreate in GTK and Qt. That's not mentioning the increased amount of time and effort in maintaining those themes. Perhaps this is an area that the community will contribute to though.
Color palette adjustments alone would bring it pretty close. We can automatically generate them on the fly.