use highlight colour for any font-awesome icons in file-tree.

services/web/public/stylesheets/app/editor/file-tree.less

@@ -112,7 +112,7 @@ aside#file-tree {
 				border-right: 4px solid @link-color;
 				font-weight: bold;
 				padding-right: 32px;
-				i.fa-folder-open, i.fa-folder, i.fa-file, i.fa-image, i.fa-file-pdf-o {
+				i.fa-folder-open, i.fa {
 					color: @link-color;
 				}
 				.entity-menu-toggle {