header border config

assets/sass/_theme/_configuration.sass

@@ -172,6 +172,7 @@ $header-logo-height-desktop: $header-logo-height !default
 $header-height: 99px !default
 $header-height-desktop: 74px !default
 $header-sticky-invert-logo: false !default
+$header-border-bottom-width: 1px !default
 
 // Navs
 $body-overlay-color: rgba(0, 0, 0, 0.3) !default

assets/sass/_theme/design-system/header.sass

 header#document-header
     background: $header-background
-    border-bottom: 1px solid $color-border
+    border-bottom: $header-border-bottom-width solid $color-border
     color: $header-color
     position: fixed
     left: 0