Add a --header-box-shadow theme css variable

Change-Id: Iecd262f2cb43ecd4f95c6f52cd21c5d0236be7a1
diff --git a/polygerrit-ui/app/elements/gr-app-element.html b/polygerrit-ui/app/elements/gr-app-element.html
index ca60de4..046e5ff 100644
--- a/polygerrit-ui/app/elements/gr-app-element.html
+++ b/polygerrit-ui/app/elements/gr-app-element.html
@@ -75,6 +75,7 @@
         border-right: 0;
         border-left: 0;
         border-top: 0;
+        box-shadow: var(--header-box-shadow);
       }
       footer {
         background: var(--footer-background, var(--footer-background-color, #eee));
diff --git a/polygerrit-ui/app/styles/themes/app-theme.html b/polygerrit-ui/app/styles/themes/app-theme.html
index 948aa1a..104a5f5 100644
--- a/polygerrit-ui/app/styles/themes/app-theme.html
+++ b/polygerrit-ui/app/styles/themes/app-theme.html
@@ -107,6 +107,7 @@
   --header-background-color: #eee;
   --header-border-bottom: 1px solid var(--border-color);
   --header-border-image: '';
+  --header-box-shadow: none;
   --header-padding: 0 var(--spacing-l);
   --header-icon-size: 0em;
   --header-icon: none;