fix nav-dropdown styles on mobile (vuejs#358)

Author: vuejs-jp-bot

themes/vue/source/css/_header.styl

@@ -31,6 +31,23 @@ body.docs
     position: relative
     margin: 0 .6em
 
+  .nav-dropdown
+    .nav-link
+      &:hover, &.current
+        border-bottom: none
+      &.current
+        &::after
+          content: ""
+          width: 0
+          height: 0
+          border-left: 5px solid $green
+          border-top: 3px solid transparent
+          border-bottom: 3px solid transparent
+          position: absolute
+          top: 50%
+          margin-top: -4px
+          left: 8px
+
 .nav-link
   padding-bottom: 3px
   &:hover, &.current