[ERR_MODULE_NOT_FOUND] when consuming custom theme

[silvester-pari]

Describe the bug

When creating a custom theme following the docs, and then installing it in another VitePress page, it throws an error at docs:build (working fine on docs:dev:

Cannot find module '/home/silvester/Desktop/temp3/page/node_modules/vitepress/dist/client/theme-default/without-fonts' imported from /home/silvester/Desktop/temp3/page/node_modules/vitepress/dist/client/theme-default/index.js
Error [ERR_MODULE_NOT_FOUND]: Cannot find module '/home/silvester/Desktop/temp3/page/node_modules/vitepress/dist/client/theme-default/without-fonts' imported from /home/silvester/Desktop/temp3/page/node_modules/vitepress/dist/client/theme-default/index.js
    at finalizeResolution (node:internal/modules/esm/resolve:265:11)
    at moduleResolve (node:internal/modules/esm/resolve:933:10)
    at defaultResolve (node:internal/modules/esm/resolve:1169:11)
    at ModuleLoader.defaultResolve (node:internal/modules/esm/loader:542:12)
    at ModuleLoader.resolve (node:internal/modules/esm/loader:510:25)
    at ModuleLoader.getModuleJob (node:internal/modules/esm/loader:239:38)
    at ModuleWrap.<anonymous> (node:internal/modules/esm/module_job:96:40)
    at link (node:internal/modules/esm/module_job:95:36)

import MyTheme from 'my-theme';

export default {
  extends: MyTheme,
}

Reproduction

Reproduction repository: https://github.com/silvester-pari/vitepress-custom-theme-error
(Follow instructions in Readme)

Expected behavior

When extending a Theme based on the DefaultTheme, it should work.

System Info

System:
    OS: Linux 6.8 Linux Mint 22 (Wilma)
    CPU: (8) x64 Intel(R) Core(TM) i7-8550U CPU @ 1.80GHz
    Memory: 5.49 GB / 15.39 GB
    Container: Yes
    Shell: 5.2.21 - /bin/bash
  Binaries:
    Node: 20.18.1 - ~/.nvm/versions/node/v20.18.1/bin/node
    npm: 10.8.2 - ~/.nvm/versions/node/v20.18.1/bin/npm
  Browsers:
    Chrome: 131.0.6778.204

Additional context

No response

Validations

• Check if you're on the latest VitePress version.
• Follow our Code of Conduct
• Read the docs.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.

[brc-dd]

Add your theme's package name to vite.ssr.noExternal in vitepress config.

It needs fixing on our part to work, but it might take time and noExternal should work fine in meanwhile.

[silvester-pari]

Thank you, I can confirm it works with:

// page/.vitepress/config.mjs
vite: {
  ssr: {
    noExternal: ['my-theme']
  }
}