Answer
The directory file structure shown before works fine. I've solved the problem just adding the necessary js and css on standard_layout.html.twig
{% block stylesheets %}
...
<!-- custom stylesheets -->
<link rel="stylesheet" href="{{ asset('bundles/mybundle/css/mycssfile.css') }}" media="all">
{% endblock %}
{% block javascripts %}
...
<!-- custom js -->
<script src="{{ asset('bundles/mybundle/js/myscriptfile.css') }}"></script>
{% endblock %}