The last update of the theme has the menu IDs and Classes renamed. This is done to avoid having duplicate IDs whenever a custom menu is given a name of “main”. This really should have been done from the start. Sorry about that.
This will only affect users who have their own custom css that styles the menus. If you have some custom css saved on frontier options or on a child-theme’s style.css that modifies the menus, you’ll have to replace each instance of the id or class with the new one. Ex. Replace every #menu-main with #nav-main