searxng/searx/templates/default/categories.html

<div id="categories">
    <div id="categories_container">
{% for category in categories %}
        <div class="checkbox_container">
             <input type="checkbox" id="checkbox_{{ category|replace(' ', '_') }}" name="category_{{ category }}" {% if category in selected_categories %}checked="checked"{% endif %} /><label for="checkbox_{{ category|replace(' ', '_') }}">{{ _(category) }}</label>
        </div>
{% endfor %}
    </div>
</div>