Nav dropdown transparent fixed then change to solid upon scroll

Hi all

In Squarespace 7.1, there is already build in options to choice both “fixed position” and “transparent” in EDIT SITE HEADER. 

BUT, it doesn’t work. When the main nav is transparent, the dropdowns have a solid background. If I use code to make the dropdowns transparent, they don't turn solid upon scroll like my main nav does.

So how do I code to get transparent dropdown menu when it’s fixed and solid upon scroll to match my navigation settings? 

You're amazing. Thank you so much! 🙂