Having looked into the code and templates for individual controls, I am noticing a lot of hard coded values, which before WinUI, would have been ThemeResources or StaticResources referring to SolidColorBrushes, which called back on the core System colours which are part of the ColorPaletteResources system.

I know there is conditional XAML which compiles into ResourceDictionaries for each supported version of Windows. But those hard coded colours still appear in these XAML files.