Use less variable for header bg color

IPython/html/static/tree/less/tree.less

@@ -186,7 +186,7 b' ul#new-menu {'
             margin-top: 15px;
 
             .panel-heading {
-                background-color: #eee; /* @page-backdrop-color */
+                background-color: @page-backdrop-color;
                 padding-top: @dashboard_tb_pad;
                 padding-bottom: @dashboard_tb_pad;
                 padding-left: @dashboard_lr_pad;