diff --git a/docs/guide/custom-themes.md b/docs/guide/custom-themes.md
index c2f75cec45..75aeaefc8d 100644
--- a/docs/guide/custom-themes.md
+++ b/docs/guide/custom-themes.md
@@ -66,6 +66,20 @@ The compiled content of the current `.md` file being rendered will be available
 </template>
 ```
 
+## Theme Level Enhancements
+
+Themes can extend the Vue app that VuePress uses by exposing an `index.js` file at the root of the theme. The file should `export default` a hook function which will receive an object containing some app level values. You can use this hook to install additional Vue plugins, register global components, or add additional router hooks:
+
+``` js
+export default ({
+  Vue, // the version of Vue being used in the VuePress app
+  options, // the options for the root Vue instance
+  router // the router instance for the app
+}) => {
+  // ...apply enhancements to the app
+}
+```
+
 ## Using Theme from a Dependency
 
 Themes can be published on npm in raw Vue SFC format as `vuepress-theme-xxx`.
diff --git a/lib/app/app.js b/lib/app/app.js
index 4db19c1acc..1b7f708017 100644
--- a/lib/app/app.js
+++ b/lib/app/app.js
@@ -7,6 +7,7 @@ import NotFound from '@notFound'
 import { routes } from '@temp/routes'
 import { siteData } from '@temp/siteData'
 import enhanceApp from '@temp/enhanceApp'
+import themeEnhanceApp from '@temp/themeEnhanceApp'
 
 // suggest dev server restart on base change
 if (module.hot) {
@@ -75,7 +76,8 @@ export function createApp () {
   })
 
   const options = {}
-
+  
+  themeEnhanceApp({ Vue, options, router })
   enhanceApp({ Vue, options, router })
 
   const app = new Vue(
diff --git a/lib/prepare.js b/lib/prepare.js
index bb74626ad1..1f86c2e2f1 100644
--- a/lib/prepare.js
+++ b/lib/prepare.js
@@ -53,15 +53,21 @@ if (!Object.assign) Object.assign = require('object-assign')`
     await writeTemp(`override.styl`, hasUserOverride ? `@import(${JSON.stringify(overridePath)})` : ``)
   }
 
+  async function writeEnhanceTemp (destName, srcPath, isEnhanceExist) {
+    await writeTemp(
+      destName,
+      isEnhanceExist
+        ? `export { default } from ${JSON.stringify(srcPath)}`
+        : `export default function () {}`
+    )
+  }
+
   // 6. handle enhanceApp.js
-  const enhancePath = path.resolve(sourceDir, '.vuepress/enhanceApp.js')
-  const hasEnhancePath = fs.existsSync(enhancePath)
-  await writeTemp(
-    'enhanceApp.js',
-    hasEnhancePath
-      ? `export { default } from ${JSON.stringify(enhancePath)}`
-      : `export default function () {}`
-  )
+  const enhanceAppPath = path.resolve(sourceDir, '.vuepress/enhanceApp.js')
+  writeEnhanceTemp('enhanceApp.js', enhanceAppPath, fs.existsSync(enhanceAppPath))
+
+  // 7. handle the theme index.js
+  writeEnhanceTemp('themeEnhanceApp.js', options.themeApp, fs.existsSync(options.themeApp))
 
   return options
 }
@@ -153,6 +159,11 @@ async function resolveOptions (sourceDir) {
     } else {
       options.notFoundPath = path.resolve(__dirname, 'default-theme/NotFound.vue')
     }
+
+    const themeApp = path.resolve(themeDir, 'index.js')
+    if (fs.existsSync(themeApp)) {
+      options.themeApp = themeApp
+    }
   }
 
   // resolve pages