def is_dark_terminal_background(cls):
        """
        :return: Whether we have a dark Terminal background color, or None if unknown.
            We currently just check the env var COLORFGBG,
            which some terminals define like "<foreground-color>:<background-color>",
            and if <background-color> in {0,1,2,3,4,5,6,8}, then we have some dark background.
            There are many other complex heuristics we could do here, which work in some cases but not in others.
            See e.g. `here <https://stackoverflow.com/questions/2507337/terminals-background-color>`__.
            But instead of adding more heuristics, we think that explicitly setting COLORFGBG would be the best thing,
            in case it's not like you want it.
        :rtype: bool|None
        """
        if os.environ.get("COLORFGBG", None):
            parts = os.environ["COLORFGBG"].split(";")
            try:
                last_number = int(parts[-1])
                if 0 <= last_number <= 6 or last_number == 8:
                    return True
                else:
                    return False
            except ValueError:  # not an integer?
                pass
        return None
:return: Whether we have a dark Terminal background color, or None if unknown.
            We currently just check the env var COLORFGBG,
            which some terminals define like "<foreground-color>:<background-color>",
            and if <background-color> in {0,1,2,3,4,5,6,8}, then we have some dark background.
            There are many other complex heuristics we could do here, which work in some cases but not in others.
            See e.g. `here <https://stackoverflow.com/questions/2507337/terminals-background-color>`__.
            But instead of adding more heuristics, we think that explicitly setting COLORFGBG would be the best thing,
            in case it's not like you want it.
        :rtype: bool|None