Adding offset to the index navigation scroll

Site URL: https://rose-octahedron-r9ek.squarespace.com/respire

I am using Squarespace 7.0 Brine template with index navigation and fixed header that becomes smaller using jQuery. 

Problem: Every time I click on index navigation to scroll down to a page, it off by the size of head due to its fixed nature. Please let me know if something can be done.  

Link: https://rose-octahedron-r9ek.squarespace.com/respire

Password: 12345