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$ php bin/console make:controller DefaultController

// ...
class DefaultController extends Controller
{
    /**
     * @Route("/")
     */
    public function new(Request $request)
    {

        // ...
        return $this->render('default/new.html.twig', [
            'default' => $default->createView(),
        ]);
    }
}

// ...
  /**
   * @Route("/")
   */
  public function new(Request $request)
  {
    // ...
    return $this->render('base.html.twig', [
        'default' => $default->createView(),
    ]);
  }
// ...

<html>
  <head>
     <title>{{ title }}</title>

      {% block stylesheets %}
           <link href="{{ asset('css/base.css') }}" rel="stylesheet"/>
        {% endblock %}
  </head>

  <body>
    {% block header %}
       <div>Верстка общего хидера</div>
    {% endblock %}

    {% block content %}{% endblock %}

    {% block footer %}
       <div>Верстка общего футера</div>
    {% endblock %}

    {% block javascripts %}
        <script src="{{ asset('js/base.js') }}"></script>
    {% endblock %}
  </body>
</html>

{% extends 'base.html.twig' %}

 {% block stylesheets %}
     {{ parent() }}
      <link href="{{ asset('css/new.css') }}" rel="stylesheet"/>
 {% endblock %}

 {% block javascripts %}
     {{ parent() }}
      <script src="{{ asset('js/new.js') }}"></script>
 {% endblock %}

 {% block content %}
     <div>разметка контента</div>
 {% endblock %}