The theme is designed with a menu appearance where all menu items fit in the allotted space (on the target screen sizes). If this does not happen, then the menu will overflow on top of the header image / slider image (depending on the site section viewed).
This can be avoided by ensuring the header content (logo/title, menu, socials) fit in the available width.
In the Plus edition of the theme it is additionally possible to reconfigure the screen width at which the desktop menu (that could be overflowing) gets replaced by the smaller mobile menu.