Styling the sticky nav/header (7.1)

Hello,

I'd like the sticky header on the homepage to have a solid white background and turn the menu items and shopping cart to black, while still keeping the header on the landing transparent as it is right now.

https://pelican-chiton-xwy4.squarespace.com/

Has anybody managed to do that or is willing to help with the code? My site is running in 7.1

Thanks!

Xavier