replace stripes with hover highlight

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

@@ -82,8 +82,8 b' ul.breadcrumb {'
     background-color: @table-border-color;
   };
   a {text-decoration: none;}
-  &:nth-child(odd) {
-    background-color: @list_stripe_color;
+  &:hover {
+    background-color: darken(white,2%);
   }
 }