teal top-level property Null safety

Implementation

NBColor teal = const NBColor(
    0xff14b8a6,
    <String, Color>{
      'teal.50 ': Color(0xfff0fdfa),
      'teal.100': Color(0xffccfbf1),
      'teal.200': Color(0xff99f6e4),
      'teal.300': Color(0xff5eead4),
      'teal.400': Color(0xff2dd4bf),
      'teal.500': Color(0xff14b8a6),
      'teal.600': Color(0xff0d9488),
      'teal.700': Color(0xff0f766e),
      'teal.800': Color(0xff115e59),
      'teal.900': Color(0xff134e4a),
    },
    'teal');