feat($theme-default): unify the text color of sidebar heading text and root-level sidebar link

Author: ulivz

packages/@vuepress/theme-default/components/SidebarGroup.vue

@@ -97,7 +97,7 @@ export default {
       border-left none
 
 .sidebar-heading
-  color #999
+  color $textColor
   transition color .15s ease
   cursor pointer
   font-size 1.1em