Problem with header border.

Hi,

I need help with this, the bottom-border on the header is always switchching to yellow even if I force with css to be another color.
I don't understand why this keeps happening.

Example mode

Edit Mode

I want the line to be like it is in the edit mode.

Thank you for the help

Hi, can you share URL site?

yes, no problem

https://crocodile-magnolia-9e5r.squarespace.com/

Thank you so much!