content/static: update banner background color

To make the edge of banners and the header more distinct.

Change-Id: If77eada5a208579855800e1577293a8f84a846f8
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/307356
Trust: Jamal Carvalho <jamal@golang.org>
Run-TryBot: Jamal Carvalho <jamal@golang.org>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Julie Qiu <julie@golang.org>
diff --git a/content/static/css/unit_header.css b/content/static/css/unit_header.css
index d419847..105ff5f 100644
--- a/content/static/css/unit_header.css
+++ b/content/static/css/unit_header.css
@@ -219,7 +219,7 @@
 }
 .UnitHeader-banner--majorVersion,
 .UnitHeader-banner--redirected {
-  background-color: var(--gray-10);
+  background-color: var(--gray-9);
 }
 .UnitHeader-banner--deprecated,
 .UnitHeader-banner--retracted {