Horizontal line under Top Navigation Bar

Site URL: https://ray-dory-c4h3.squarespace.com/config/

Hello! Thank you in advanced to all who help! I cannot for the life of me get a line or "border-bottom" under by header. I am trying to achieve a navigation bar like this: https://www.hilma.co/ 

I have tried 

.Header { border-bottom: 5px solid #b0b579 ;}  

This code does nothing on my site as well as

.footer { border-top: 5px solid #b0b579 ;}  

does nothing. PLEASE SEND HELP

I use this code in custom css for a black line under active link. You can make it larger by adjusting the px and change the color if you wish. 

main-nav ul li.active-link a,

.main-nav ul li.active-link a:visited {
     border-bottom: 4px solid #000000;
 }

This one turns a link border on all the time on hover, I have not tried it myself:

#nav > ul > li:hover {border-color:rgba(192,192,192,0.5)} 

You can convert/choose a different color here:

https://convertingcolors.com/rgb-color-192_192_192.html#:~:text=The RGB color 192%2C 192,the color name is silver.

Your site is private. Can you setup password & share url? We can help easier