Fix Menu[dark] item background, close #4414 again

components/menu/demo/horizontal.md

 ---
 order: 0
-title: 
+title:
   zh-CN: 顶部导航
   en-US: Top Navigation
 ---
@@ -32,7 +32,9 @@ const App = React.createClass({
   },
   render() {
     return (
-      <Menu onClick={this.handleClick}
+      <Menu
+        theme="dark"
+        onClick={this.handleClick}
         selectedKeys={[this.state.current]}
         mode="horizontal"
       >

components/menu/style/index.less

@@ -390,7 +390,7 @@
     }
   }
 
-  &-dark &-item-selected {
+  &&-dark &-item-selected {
     background-color: transparent;
   }