getgrav_grav/system/templates/partials/messages.html.twig

{% set status_mapping = {'info':'green', 'error': 'red', 'warning': 'yellow'} %}

{% if grav.messages.all %}
    <div id="messages">
        {% for message in grav.messages.fetch %}

            {% set scope = message.scope|e %}
            {% set color = status_mapping[scope] %}

            <div class="notices {{ scope }} {{ color }}"><p>{{ message.message|raw }}</p></div>

        {% endfor %}
    </div>
{% endif %}