style: add color

Author: artdong

src/org_tree.less

@@ -36,6 +36,7 @@
   .org-tree-node-label-inner {
     padding: 10px 15px;
     text-align: center;
+    color: black;
     border-radius: 3px;
     box-shadow: 0 1px 5px rgba(0, 0, 0, .15);
   }