Set new theme to use. Uses a direct tk call to allow usage
        of the themes supplied with this package.

        :param theme_name: name of theme to activate

def set_theme(self, theme_name):
        """
        Set new theme to use. Uses a direct tk call to allow usage
        of the themes supplied with this package.

        :param theme_name: name of theme to activate
        """
        package = theme_name if theme_name not in self.PACKAGES else self.PACKAGES[theme_name]
        self.tk.call("package", "require", "ttk::theme::{}".format(package))
        self.tk.call("ttk::setTheme", theme_name)