searxng/searx/templates/categories.html

{% 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(' ', '_') }}" class="cb"></label><label for="checkbox_{{ category|replace(' ', '_') }}">{{ category }}</label>
    </div>
{% endfor %}