fix: active side arrow not middle align (#508).

Author: ulivz

Diff for: lib/default-theme/DropdownLink.vue

@@ -95,7 +95,7 @@ export default {
             border-top 3px solid transparent
             border-bottom 3px solid transparent
             position absolute
-            top calc(1rem - 2px)
+            top calc(50% - 2px)
             left 9px
       &:first-child h4
         margin-top 0

Diff for: lib/default-theme/Sidebar.vue

@@ -97,6 +97,8 @@ function resolveOpenGroupIndex (route, items) {
   .sidebar
     .nav-links
       display block
+      .dropdown-wrapper .nav-dropdown .dropdown-item a.router-link-active::after
+        top calc(1rem - 2px)
     .sidebar-links
       padding 1rem 0
 </style>