Hamburger menu not working on Zita theme

I’m evaluating the Zita theme and found an issue when accessing the web site with a tablet or phone. It auto converts the menu (which is showing under the title in the center on a laptop) to a hamburger menu on these devices. It took me a while to figure out what was happening, but was able to change the hamburger colours so that the meny icon is showing. However, when you click the menu icon, a blank white page appears – no menu options are displaying.

Not sure if anyone else has had this issue and know how to correct it?

Thanks

The page I need help with: [log in to see the link]