Importing default composables also imports default theme's styles

[mrbbot]

Describe the bug
When using a custom theme, importing composables like useRoute from "vitepress" in the Layout component also imports the styles for the default theme.

To Reproduce

1. Follow the Getting Started guide to create a new site
2. Create docs/.vitepress/theme/Layout.vue with the following contents:

<template>
  <Content />
</template>

<script setup>
// import { useRoute } from "vitepress";
// const route = useRoute();
</script>

3. Create docs/.vitepress/theme/index.js with the following contents:

import Layout from "./Layout.vue";

export default {
  Layout,
};

4. Run yarn docs:dev and open http://localhost:3000. Note the heading doesn't have any styles applied to it:

5. Uncomment the lines in the <script setup> section in docs/.vitepress/theme/Layout.vue. Note the heading now has the default styles applied to it, even though we only imported useRoute:

Expected behavior
The heading should remain unstyled since we aren't explicitly importing any styles.

System Info

• vitepress version: v0.9.2
• vite version: v1.0.0-rc.13
• Node version: v12.20.0
• OS version: macOS Catalina 10.15.7

Additional context
I had a quick look at the Vitepress code and it looks like the problem might be to do with this line:

vitepress/src/client/app/exports.ts

Line 29 in 309aa7a

export { default as defaultTheme } from '/@default-theme/index'

Adding the following to docs/.vitepress/config.js:

module.exports = {
  alias: {
    "/@default-theme/index": "/@theme/empty.js",
  },
};

...and creating an empty file docs/.vitepress/theme/empty.js prevents the default styles being applied.

[harlan-zw]

This one also tripped me up when I upgraded from version 0.8.1 to latest, I got around it by importing the exactly what I needed.

This causes the styles from the default theme to be applied:

import { useSiteData } from 'vitepress'

This does what we'd expect without the leakage:

import { useSiteData } from 'vitepress/dist/client/app/composables/siteData'

With the suggestion of importing for the default theme from this issue #190 would it make sense / be possible to remove the default theme export from the types/index.d.ts file? (assuming that is the culript)

[kiaking]

This issue is fixed in the latest code. Importing things like useSiteData will not include styles. So the styles are only included when you actually do import DefaultTheme from 'vitepress/theme' 👍