Set included in dialog header background for scrolling

Bug: Issue 8982
Change-Id: Ia9f67c36a54c09e016d0999005ba6227c992b41c
diff --git a/polygerrit-ui/app/elements/change/gr-included-in-dialog/gr-included-in-dialog.html b/polygerrit-ui/app/elements/change/gr-included-in-dialog/gr-included-in-dialog.html
index cf79a31..b824f1c 100644
--- a/polygerrit-ui/app/elements/change/gr-included-in-dialog/gr-included-in-dialog.html
+++ b/polygerrit-ui/app/elements/change/gr-included-in-dialog/gr-included-in-dialog.html
@@ -30,6 +30,7 @@
         padding: 4.5em 1em 1em 1em;
       }
       header {
+        background-color: var(--dialog-background-color);
         border-bottom: 1px solid var(--border-color);
         left: 0;
         padding: 1em;