In order to develop plonetheme.barceloneta you need to execute on the root of the package:
npm install
After executing this you can run the following command to watch for any scss changes in the path plonetheme.barceloneta/scss:
npm run watch
This will make sure that the many .scss files are compiled to .css on the fly and then copied over to the theme.
If you simply want to compile all resources before a release run:
npm run build
See plonetheme.barceloneta/plonetheme/barceloneta/theme/rules.xml for the relevant Diazo theme rules.
Read plonetheme.barceloneta/plonetheme/barceloneta/theme/roboto/README.rst if you need to update the shipped fonts.