The color and weight of the chip's outline.
Defaults to the border side in the ambient ChipThemeData. If the theme border side resolves to null and ThemeData.useMaterial3 is true, then BorderSide with a ColorScheme.outline color is used when the chip is enabled, and BorderSide with a ColorScheme.onSurface color with an opacity of 0.12 is used when the chip is disabled. Otherwise, it defaults to null.
final BorderSide? side;