It worked! Thank you very much!
oldlongisland.org/WP-OLI-JW
The site is under construction.
I used the .menu class because it directly controlled the menu rather than the header, which I didn’t want visible.
All CSS edits I made were done to reposition the menu for each screen resolution (centred and different formatting as needed), aside the lines to remove the black header, but maintain the menu.
It keeps the header (black bar along the top) in place, but .menu class is still sticky to the top of the screen when you scroll. I tried placing position: absolute !important; within the .menu class but it has no effect.