Problems getting file id in start script


Currently I use BonitaStudio 7.7.4 and I have a human task with a form with multiple data to which a file can be loaded.

Create a multiple document that takes its value from the variable declared in the contract.
I want for each multiple record to store the id of the document so that in the next task I can show each multiple data in a table with its associated document.

I am using this code within each:
Document doc = apiAccessor.getProcessAPI().getDocument(processInstanceId, currentComisionMensualDetalleInput.documentoAsociado).getId();

BonitaSoft portal .css files. Where can I find them?

Do I need to change the BonitaSoft Portal Layout but can not find the .css or .js files, is there a default directory where I can access these files?