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

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.