code: def set_theme(self, theme_name):
          
          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)
docs: 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