Add background color to group headers in change list

Bug: Issue 5394
Change-Id: Ia27641e2e71921c014d20b7f3d5632412cb48af1
diff --git a/polygerrit-ui/app/elements/change-list/gr-change-list/gr-change-list.html b/polygerrit-ui/app/elements/change-list/gr-change-list/gr-change-list.html
index 8a95fa8..362f101 100644
--- a/polygerrit-ui/app/elements/change-list/gr-change-list/gr-change-list.html
+++ b/polygerrit-ui/app/elements/change-list/gr-change-list/gr-change-list.html
@@ -35,6 +35,13 @@
       th {
         text-align: left;
       }
+      .groupHeader {
+        background-color: #eee;
+        border-top: 1em solid #fff;
+      }
+      .headerRow + tr {
+        border: none;
+      }
     </style>
     <style include="gr-change-list-styles"></style>
     <table id="changeList">