grav-admin-first/user/plugins/form/templates/forms/default/toggleable.html.twig

<span class="checkboxes toggleable" data-grav-field="toggleable" data-grav-field-name="{{ field_name }}">
    <input type="checkbox"
           id="toggleable_{{ field.name }}"
           name="toggleable_{{ field_name }}"
           value="1"
           {% if checked %}checked="checked"{% endif %}
    >
    <label for="toggleable_{{ field.name }}"></label>
</span>