How to add a line to bottom of header?

Site URL: https://www.nickwaldinger.com/

Hello,

I'm trying to add a 1 px white line to the bottom of my header (see screenshots attached; "NoBorder" is how it currently looks and "WithBorder" is how I'm trying to get it to look). I'd like it to go the full width of my site and be on all pages. My website is www.nickwaldinger.com and my site password is "create".

Could someone help me?

Thank you!

Add to Design > Custom CSS

/* Header line */
header.Header {
    border-bottom: 1px solid white;
}