Ok, seems that javascript files are loading correctly after a server restart, but still can't add css to the admin part.
Javascript to the admin part is including creating an admin.js file with the appropiate require command, and put that file in my extension assets/javascripts directory That way the javascript code is only include in the admin part, but now in the website (as it is not needed there)
As the admin part of refinerycsm does not includes application.css, I'm trying to to a similar trick with the refinery.css file, which does seems to be included in the admin part, but it isn't working.
So I change my question: how can I add css assets only in the admin part of refinerycms?