yellow top-level property Null safety

Implementation

NBColor yellow = const NBColor(
    0xffeab308,
    <String, Color>{
      'yellow.50 ': Color(0xfffefce8),
      'yellow.100': Color(0xfffef9c3),
      'yellow.200': Color(0xfffef08a),
      'yellow.300': Color(0xfffde047),
      'yellow.400': Color(0xfffacc15),
      'yellow.500': Color(0xffeab308),
      'yellow.600': Color(0xffca8a04),
      'yellow.700': Color(0xffa16207),
      'yellow.800': Color(0xff854d0e),
      'yellow.900': Color(0xff713f12),
    },
    'yellow');