use usePreferredDark with appearance: force-auto

src/client/app/data.ts

@@ -1,5 +1,5 @@
 import siteData from '@siteData'
-import { useDark } from '@vueuse/core'
+import { useDark, usePreferredDark } from '@vueuse/core'
 import {
   computed,
   inject,
@@ -79,6 +79,8 @@ export function initData(route: Route): VitePressData {
   const isDark =
     appearance === 'force-dark'
       ? ref(true)
+      : appearance === 'force-auto'
+      ? usePreferredDark()
       : appearance
       ? useDark({
           storageKey: APPEARANCE_KEY,