Rose Madder is a dark mode theme, it will switch to it if you use it, and stop using it if you switch to light mode.
I’m not sure where you saw the stock user interface change colour in Mac screenshots though. The only thing I can think of is that some older screenshots will display the old blue editor highlight when the view is split. That is now driven by the accent colour you choose in the system Appearance settings pane.
Also, from your screenshot it looks like you may have the Reduce transparency accessibility setting enabled, which will suppress the wallpaper colour showing up all over the place—and in particular to this demonstration, the title bar. With that off the menu would be a rainbow.
Scrivener largely ignores the wallpaper anyway though, so if you prefer that setting on (as I do), you might as well keep it.