Fixed the wrong CSS styles applied since f86d7f2d15304654a7af857879bb5e258f146625

Since the f86d7f2d15304654a7af857879bb5e258f146625 commit, the CSS stylesheets
were splitted into smaller CSS fragments. The problem is that blocks defined in
the included templates aren't available in the parent template:

{# parent.twig #}
{{ include("child.twig") }}

{# child.twig #}
{% block my_block %}
  ...
{% endblock %}

The result is that `my_block` isn't available in parent.twig. The solution
requires the use of the `use` Twig tag:

{# parent.twig #}
{% use "child.twig" %}
{{ block("my_block") }}

{# child.twig #}
{% block my_block %}
  ...
{% endblock %}