Build instruction does not build/create main.css expected by demo application #346

Hi,

I am new to this library and wanted to play with it. Following recipe resulted in issue:
Environment:

• OS - Windows 10
• Shell - Powershell
  Build process:

1. build.cmd build-demo
2. http-server .\build\ - I am using http-server package from npm

main.css was missing.

Workaround:

1. Open the demo-application linked in README
2. Copy the main.css file to build folder
3. Refresh the page

Rendering worked.