vitepress/docs/guide/customization.md

Customization

You can develop your custom theme by adding the .vitepress/theme/index.js file.

.
├─ docs
│  ├─ .vitepress
│  │  ├─ theme
│  │  │  └─ index.js
│  │  └─ config.js
│  └─ index.md
└─ package.json

In .vitepress/theme/index.js, you must export theme object and register your own theme layout. Let's say you have your own Layout component like this.

<!-- .vitepress/theme/Layout.vue -->
<template>
  <h1>Custom Layout!</h1>
  <Content/><!-- make sure to include markdown outlet -->
</template>

Then include it in .vitepress/theme/index.js.

// .vitepress/theme/index.js
import Layout from './Layout.vue'

export default {
  Layout,
  NotFound: () => 'custom 404', // <- this is a Vue 3 functional component
  enhanceApp({ app, router, siteData }) {
    // app is the Vue 3 app instance from `createApp()`. router is VitePress'
    // custom router. `siteData`` is a `ref`` of current site-level metadata.
  }
}

If you want to extend the default theme, you can import it from vitepress/dist/client/theme-default.

// .vitepress/theme/index.js
import DefaultTheme from 'vitepress/dist/client/theme-default'

export default {
  ...DefaultTheme
}