docs: persist color themes (#18102)

docs/.vitepress/config/head.ts

@@ -152,9 +152,3 @@ gtag('config', 'UA-175337989-1');`,
     `,
   ],
 ]
-
-head.push([
-  'script',
-  {},
-  fs.readFileSync(path.resolve(vpRoot, 'dark-mode.js'), 'utf-8'),
-])

docs/.vitepress/dark-mode.js

@@ -1,10 +0,0 @@
-;(() => {
-  const saved = localStorage.getItem('el-theme-appearance')
-  if (
-    saved === 'auto'
-      ? window.matchMedia(`(prefers-color-scheme: dark)`).matches
-      : saved === 'dark'
-  ) {
-    document.documentElement.classList.add('dark')
-  }
-})()

docs/.vitepress/vitepress/composables/dark.ts

@@ -1,7 +1,7 @@
 import { useDark, useToggle } from '@vueuse/core'
 
 export const isDark = useDark({
-  storageKey: 'el-theme-appearance',
+  storageKey: 'vitepress-theme-appearance',
 })
 
 export const toggleDark = useToggle(isDark)