docs: def _get_format_callable(term, color, back_color):
          """Get string-coloring callable
  
          Get callable for string output using ``color`` on ``back_color``
              on ``term``
  
          :param term: blessings.Terminal instance
          :param color: Color that callable will color the string it's passed
          :param back_color: Back color for the string
          :returns: callable(s: str) -> str
          """
          if isinstance(color, str):
              ensure(
                  any(isinstance(back_color, t) for t in [str, type(None)]),
                  TypeError,
                  "back_color must be a str or NoneType"
              )
              if back_color:
                  return getattr(term, "_".join(
                      [color, "on", back_color]
                  ))
              elif back_color is None:
                  return getattr(term, color)
          elif isinstance(color, int):
              return term.on_color(color)
          else:
              raise TypeError("Invalid type {} for color".format(
                  type(color)
              ))