Reduce width of header border

Hi, I have added a border to the header section of this new website. I am only wanting it to appear on the bottom section, which I have managed to achieve through the available settings on Squarespace. However, I don't want the line to be full width. Is there a way to achieve this with some code? I've added an image of what I hope to achieve. 

Site URL: https://hyperboloid-bellflower-y6t5.squarespace.com

Password: Test

Thanks,

Nicola

First remove Squarespaces' border, then try this instead:

.header-display-desktop
{border-bottom: 1px solid rgba(255,255,255,.5);}

.header.shrink
.header-display-desktop
{border-bottom: 0px solid #ffffff;}