You can enable a custom theme by adding the `.vitepress/theme/index.js` file (the "theme entry file").
```
.
├─ docs
│ ├─ .vitepress
│ │ ├─ theme
│ │ │ └─ index.js
│ │ └─ config.js
│ └─ index.md
└─ package.json
```
A VitePress custom theme is simply an object containing three properties and is defined as follows:
```ts
interface Theme {
Layout: Component // Vue 3 component
NotFound?: Component
enhanceApp?: (ctx: EnhanceAppContext) => void
}
interface EnhanceAppContext {
app: App // Vue 3 app instance
router: Router // VitePress router instance
siteData: Ref<SiteData>
}
```
The theme entry file should export the theme as its default export:
```js
// .vitepress/theme/index.js
import Layout from './Layout.vue'
export default {
Layout,
// this is a Vue 3 functional component
NotFound: () => 'custom 404',
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.
}
}
```
...where the `Layout` component could look like this:
```vue
<!-- .vitepress/theme/Layout.vue -->
<template>
<h1>Custom Layout!</h1>
<!-- this is where markdown content will be rendered -->
<Content/>
</template>
```
The default export is the only contract for a custom theme. Inside your custom theme, it works just like a normal Vite + Vue 3 application. Do note the theme also needs to be [SSR-compatible](./using-vue#browser-api-access-restrictions).
To distribute a theme, simply export the object in your package entry. To consume an external theme, import and re-export it from the custom theme entry:
```js
// .vitepress/theme/index.js
import Theme from 'awesome-vitepress-theme'
export default Theme
```
## Extending the Default Theme
If you want to extend and customize the default theme, you can import it from `vitepress/theme` and augment it in a custom theme entry. Here are some examples of common customizations:
### Registering Global Components
```js
// .vitepress/theme/index.js
import DefaultTheme from 'vitepress/theme'
export default {
...DefaultTheme,
enhanceApp({ app }) {
// register global components
app.component('MyGlobalComponent', /* ... */)
}
}
```
Since we are using Vite, you can also leverage Vite's [glob import feature](https://vitejs.dev/guide/features.html#glob-import) to auto register a directory of components.
### Customizing CSS
The default theme CSS is customizable by overriding root level CSS variables:
```js
// .vitepress/theme/index.js
import DefaultTheme from 'vitepress/theme'
import './custom.css'
export default DefaultTheme
```
```css
/* .vitepress/theme/custom.css */
:root {
--vp-c-brand: #646cff;
--vp-c-brand-light: #747bff;
}
```
See [default theme CSS variables](https://github.com/vuejs/vitepress/blob/main/src/client/theme-default/styles/vars.css) that can be overridden.
The default theme's `<Layout/>` component has a few slots that can be used to inject content at certain locations of the page. Here's an example of injecting a component into the before outline:
