content: fix UX issue with highlight above "Discover Packages"

This change modifies the CSS for the header tabs. The goal of this
change is for improved UX design and to provide a clearer connection
between tab highlight and page content. Tab position is changed from top
to bottom and tab hover color is changed from red to white.

Fixes golang/go#39847

Change-Id: I26c96ba538fbefbfa7ffbc61809fa0e5d872d725
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/275952
Trust: Jamal Carvalho <jamal@golang.org>
Trust: Julie Qiu <julie@golang.org>
Reviewed-by: Julie Qiu <julie@golang.org>
diff --git a/content/static/css/stylesheet.css b/content/static/css/stylesheet.css
index e19e52d..abb4e10 100644
--- a/content/static/css/stylesheet.css
+++ b/content/static/css/stylesheet.css
@@ -283,11 +283,11 @@
 }
 .Header-menuItem--active a:link,
 .Header-menuItem--active a:visited {
-  border-top-color: var(--turq-med);
+  border-bottom-color: var(--turq-med);
   font-weight: bold;
 }
 .Header-menuItem a:hover {
-  border-top-color: var(--pink);
+  border-bottom-color: var(--white);
 }
 .Header-navOpen {
   background: no-repeat center/2rem url('/static/img/menu-24px-white.svg');