A color used on interactive elements of the theme.
This color is generally used on text and icons in buttons and tappable elements. Defaults to CupertinoColors.activeBlue.
If coming from a Material Theme and unspecified, primaryColor will be
derived from the Material ThemeData's
colorScheme.primary. However, in
iOS styling, the primaryColor is more sparsely used than in Material
Design where the primaryColor can appear on non-interactive surfaces like
the AppBar background, TextField borders etc.
final Color? primaryColor;