use IPython border color for tree view

IPython/frontend/html/notebook/static/style/style.min.css

@@ -1406,8 +1406,8 b' ul.icons-ul{list-style-type:none;text-indent:-0.7142857142857143em;margin-left:2'
 .toolbar_info{float:left;}
 .toolbar_buttons{float:right;}
 .list_header{font-weight:bold;}
-.list_container{margin-top:16px;margin-bottom:16px;border:1px solid #eeeeee;border-radius:4px;}
-.list_container>div{border-bottom:1px solid #eeeeee;}.list_container>div:hover .list-item{background-color:red;}
+.list_container{margin-top:16px;margin-bottom:16px;border:1px solid #ababab;border-radius:4px;}
+.list_container>div{border-bottom:1px solid #ababab;}.list_container>div:hover .list-item{background-color:red;}
 .list_container>div:last-child{border:none;}
 .list_item:hover .list_item{background-color:#ddd;}
 .list_container>div>span,.list_container>div>div{padding:8px;}

IPython/frontend/html/notebook/static/tree/less/tree.less

@@ -29,12 +29,12 b''
 .list_container {
   margin-top: 16px;
   margin-bottom: 16px;
-  border: 1px solid @borderColor;
+  border: 1px solid @border_color;
   border-radius: 4px;
 }
 
 .list_container > div {
-  border-bottom: 1px solid @borderColor;
+  border-bottom: 1px solid @border_color;
   &:hover .list-item{
     background-color: red;
   };