content/static: update font weight for page headings

Now that we have switched to system fonts the
headings appear too bold. Reducing the font
weight to account for the font family changes.

Change-Id: I6b16040343e56d9f595170203e03a377c665585f
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/262277
Trust: Jamal Carvalho <jamal@golang.org>
Run-TryBot: Jamal Carvalho <jamal@golang.org>
Reviewed-by: Julie Qiu <julie@golang.org>
diff --git a/content/static/css/stylesheet.css b/content/static/css/stylesheet.css
index 882b91a..a45af77 100644
--- a/content/static/css/stylesheet.css
+++ b/content/static/css/stylesheet.css
@@ -78,7 +78,12 @@
 h1,
 h2,
 h3 {
-  font-weight: bold;
+  font-weight: 600;
+}
+h4,
+h5,
+h6 {
+  font-weight: 500;
 }
 h1 {
   font-size: 1.5rem;