unselectedIconTheme property
final
The visual properties of the icon in the unselected destination.
If this field is not provided, or provided with any null properties, then a copy of the IconThemeData.fallback with a custom NavigationRail specific color will be used.
The default value is the Theme's ThemeData.iconTheme with a color of the Theme's ColorScheme.onSurface with an opacity of 0.64. Properties from this icon theme, or NavigationRailThemeData.unselectedIconTheme if this is null, are merged into the defaults.
Implementation
final IconThemeData? unselectedIconTheme;