content: add banner in support of our black community

Change-Id: I2bd5c3632a1fdbf0779e8181417950c7f9408146
Reviewed-on: https://team-review.git.corp.google.com/c/golang/discovery/+/767238
Reviewed-by: Russ Cox <rsc@google.com>
Reviewed-by: Andrew Bonventre <andybons@google.com>
diff --git a/content/static/css/sidenav.css b/content/static/css/sidenav.css
index acdc96c..cda0b07 100644
--- a/content/static/css/sidenav.css
+++ b/content/static/css/sidenav.css
@@ -14,13 +14,13 @@
   grid-template-columns: 1fr minmax(0, 3fr);
 }
 .Documentation-nav {
-  max-height: calc(100vh - 4.0625rem); /* 4.0625rem is the site header height */
+  max-height: calc(100vh - 6.511rem); /* 6.511rem is the site header height */
   overflow-x: hidden;
   overflow-y: scroll;
   padding-top: 1rem;
   padding-left: 1.75rem;
   position: sticky;
-  top: 4.0625rem;
+  top: 6.511rem;
 }
 .Documentation-toc {
   margin-top: 0;
diff --git a/content/static/css/stylesheet.css b/content/static/css/stylesheet.css
index 32d161c..eeec137 100644
--- a/content/static/css/stylesheet.css
+++ b/content/static/css/stylesheet.css
@@ -121,6 +121,40 @@
 .AutoComplete-list li[aria-selected='true'] {
   background-color: var(--gray-9);
 }
+
+.Banner {
+  /**
+   * Only show on wide viewports so the
+   * text never wraps or gets cut off.
+   */
+  display: none;
+  background-color: #000;
+}
+.Banner-inner {
+  align-items: center;
+  display: flex;
+  justify-content: space-between;
+  margin: 0 auto;
+  max-width: 75.75rem;
+  min-height: 2.5rem;
+  padding: 0.5rem 1.5rem;
+}
+.Banner-message {
+  color: #fff;
+  margin-right: 1.25rem;
+}
+.Banner-action:link,
+.Banner-action:visited {
+  color: #fff;
+  text-decoration: underline;
+  white-space: nowrap;
+}
+@media only screen and (min-width: 52rem) {
+  .Banner {
+    display: block;
+  }
+}
+
 .SearchForm[aria-expanded='false'] .AutoComplete-list,
 .Header-searchForm[aria-expanded='false'] .AutoComplete-list {
   display: none;
@@ -222,6 +256,12 @@
   pointer-events: none;
   visibility: hidden;
 }
+@media only screen and (min-width: 52rem) {
+  :target::before {
+    height: calc(5.125rem + 2.5rem);
+    margin-top: calc(-5.125rem - 2.5rem); /* Account for added height of banner. */
+  }
+}
 .Header-logoLink {
   margin-right: auto;
 }
diff --git a/content/static/html/base.tmpl b/content/static/html/base.tmpl
index 72a5da0..92195d7 100644
--- a/content/static/html/base.tmpl
+++ b/content/static/html/base.tmpl
@@ -19,6 +19,15 @@
 <title>{{if .HTMLTitle}}{{.HTMLTitle}} · {{end}}pkg.go.dev</title>
 <body class="Site{{if (.Experiments.IsActive "sidenav")}} is-withSideNav{{end}}">
 <header class="Site-header Site-header--dark">
+  <div class="Banner">
+    <div class="Banner-inner">
+      <div class="Banner-message">Black Lives Matter</div>
+      <a class="Banner-action"
+         href="https://support.eji.org/give/153413/#!/donation/checkout"
+         target="_blank"
+         rel="noopener">Support the Equal Justice Initiative</a>
+    </div>
+  </div>
   <div class="Header">
     <nav class="Header-nav">
       <a href="https://go.dev/" class="Header-logoLink">