Sets the Widget stylesheet.

def __set_style_sheet(self):
        """
        Sets the Widget stylesheet.
        """

        colors = map(
            lambda x: "rgb({0}, {1}, {2}, {3})".format(x.red(), x.green(), x.blue(), int(self.__opacity * 255)),
            (self.__color, self.__background_color, self.__border_color))
        self.setStyleSheet(self.__style.format(*colors))