In the Oxygen template I am having an issue with the top menu sub-menus, when selected, disappearing behind the header section on some computers in both Chrome and Firefox, and always on an iPad using Safari and Chrome. I don't know if my client has the latest version of the browsers, but regardless, I can't have anyone not being able to see the menu selection from the top menu. I'm not sure how to solve this issue. I've tried adding code in the custom CSS to change the z-index (since removed), but I'm not sure I'm doing it right. Any thoughts?