The theme is designed with a menu appearance where all menu items fit in the allotted space. If this does not happen, then the menu will overflow on top of the header image / slider image (depending on the site section viewed).
The solutions are to either rearrange the menu so that all the items fit in the available space at the desired site width or use custom styling to activate the mobile menu earlier (when the menu no longer fits).
Our plus themes have a built-in option to control this mobile menu activation point.