fix($theme-default): make dropdown-title's UI consistent with nav-link (#1890)

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

@@ -109,6 +109,10 @@ export default {
   .dropdown-title
     display block
     font-size 0.9rem
+    font-family inherit
+    cursor inherit
+    padding inherit
+    line-height 1.4rem
     background transparent
     border none
     font-weight 500
@@ -163,6 +167,11 @@ export default {
   .dropdown-wrapper
     &.open .dropdown-title
       margin-bottom 0.5rem
+    .dropdown-title
+      font-weight 600
+      font-size inherit
+      &:hover
+        color $accentColor
     .nav-dropdown
       transition height .1s ease-out
       overflow hidden