Use bootstrap variable for table color

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

@@ -73,7 +73,7 b' ul.breadcrumb {'
 
 .list_item {
   &:hover .list_item {
-    background-color: #ddd;
+    background-color: @table-border-color;
   };
   a {text-decoration: none;}
 }
@@ -208,7 +208,7 b' ul#new-menu {'
                     border-radius: 0px;
 
                     .list_item {
-                        border-bottom: 1px solid #ddd;
+                        border-bottom: 1px solid @table-border-color;
 
                         &:last-child {
                             border-bottom: 0px;