themes/blank/html/message.html.twig

{% extends "@themes/"~template~"/html/base.html.twig" %}
{% block stylesheets %}
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css" type="text/css" />
{% endblock %}
{% block content %}
    <div>
        {{ message|raw }}
        {% if content is defined %}
        <div>{{ content|raw }}</div>
        {% endif %}
    </div>
{% endblock %}