{% macro tree(Category) %}
    {% from _self import tree %}
    
    {% if Category.children|length > 0 and Category.hierarchy < 2 %}
        <a href="{{ url('product_list') }}?category_id={{ Category.id }}&name=&grid=deskmenu">
            <img src="{{ asset('assets/category/') }}category_{{ Category.id }}.jpg" alt="{{ Category.name }}"/>
            {{ Category.name }}
        </a>
    {% else %}
        <a href="{{ url('product_list') }}?category_id={{ Category.id }}&name=&grid=deskmenu">
           {{ Category.name }}
        </a>
    {% endif %}
        
    {% if Category.children|length > 0 %}
        <ul>
            {% for ChildCategory in Category.children %}
                <li>
                    {{ tree(ChildCategory) }}
                </li>
            {% endfor %}
        </ul>
    {% endif %}
{% endmacro %}

class="category_{{ Category.id }}" などの様に割り当てて、

.category_9:before {
    content: "\f099";
    font-family: 'Font Awesome 5 Brands';
    font-weight: 900;
    font-size: xx-large;
}