colorScheme property
final
A set of thirteen colors that can be used to derive the button theme's colors.
This property was added much later than the theme's set of highly specific colors, like ThemeData.highlightColor and ThemeData.splashColor etc.
The colors for new button classes can be defined exclusively in terms of colorScheme. When it's possible, the existing buttons will (continue to) gradually migrate to it.
Implementation
final ColorScheme? colorScheme;