Nav bar and menu items

Hi guys
Help needed please.
I am currently using the theme “Soccer” for a website I am constructing.
When I view the site on a smaller device the menu items are slipping down below the navigation bar. See for yourselves here
I am sure that this must be an easy fix in the css code, I hope so anyway?
I have not changed any of the css code for anything to have made this issue. Advice welcomed here or should I go back to the author?
Thanks all
Chris