{% macro render_form(form, action) %}
        {% from "_formhelpers.html" import render_field %}
        <div id="Product" class="variant-block" style="display: block;">
            {% from "_formhelpers.html" import render_field %}
            {% import "bootstrap/wtf.html" as wtf %}
            {{ wtf.quick_form(form, action=action, extra_classes="size-options", button_map={'submit':'light'}) }}
        </div>
    {% endmacro %}

More info. here: https://bootstrap-flask.readthedocs.io/en/stable/macros/