How to change color of menu top border

I am using index-shop-2.html and want to change the color of the light grey border bar above the menu to a darker color. I have tried everything in style.css but have not been able to. See attached PNG file with an arrow pointing to the light grey bar whose color I want to change. Please let me know what to change in style.css or elsewhere. Thank you. (I do not have a live URL - I am developing it locally on my laptop.)