Steps:
Console Output:
> @ckeditor/[email protected] build /Volumes/Work/projects/Clients/CGI/CIBA/Code/samples/ckeditor5-build-classic
> npm run create-entry-file && npm run build-ckeditor
> @ckeditor/[email protected] create-entry-file /Volumes/Work/projects/Clients/CGI/CIBA/Code/samples/ckeditor5-build-classic
> node bin/create-entry-file.js
Creating the entry file...
Done.
> @ckeditor/[email protected] build-ckeditor /Volumes/Work/projects/Clients/CGI/CIBA/Code/samples/ckeditor5-build-classic
> sh bin/build-ckeditor.sh
Building 'build/ckeditor.js'...
module.js:472
throw err;
^
Error: Cannot find module '@ckeditor/ckeditor5-theme-lark'
at Function.Module._resolveFilename (module.js:470:15)
at Function.resolve (internal/module.js:27:19)
at Object.<anonymous> (/Volumes/Work/projects/Clients/CGI/CIBA/Code/samples/ckeditor5-build-classic/webpack.config.js:65:28)
at Module._compile (module.js:571:32)
at Object.Module._extensions..js (module.js:580:10)
at Module.load (module.js:488:32)
at tryModuleLoad (module.js:447:12)
at Function.Module._load (module.js:439:3)
at Module.require (module.js:498:17)
at require (internal/module.js:20:19)
Done.
ckeditor5-theme-lark package is present in node_modules folder.
Node: v7.6.0
Npm: 4.1.2
Webpack: 3.4.1
Confirmed on my computer. A fresh clone of https://github.com/ckeditor/ckeditor5-build-classic does not work.
I can confirm that everything works when you will build the editor from the latest tag (v1.0.0-alpha.2).
So, I recommend to check out the branch from master to tag (git checkout v1.0.0-alpha.2), then call npm install and npm run build.
We will fix the docs soon.

Thanks @pomek. It works.
Hopefully, the changes that I've made to the docs in https://github.com/ckeditor/ckeditor5/commit/7d3a5f6953a1d315349fe24e857fc50a04703943 will prevent such issues in the future.
In fact, it's unlikely that we'll ever do such big changes to the building process in a near future, so it can also be a one-time thing. But I prefer to be safe.
Most helpful comment
I can confirm that everything works when you will build the editor from the latest tag (
v1.0.0-alpha.2).So, I recommend to check out the branch from master to tag (
git checkout v1.0.0-alpha.2), then callnpm installandnpm run build.We will fix the docs soon.