searxng/searx/templates/oscar/categories.html

<div id="categories" class="btn-group btn-toggle" data-toggle="buttons">
{% for category in categories %}
    <label class="btn btn-sm {% if category in selected_categories %}btn-primary active{% else %}btn-default{% endif %}" data-btn-class="primary">
        <input class="hidden" type="checkbox" id="checkbox_{{ category|replace(' ', '_') }}" name="category_{{ category }}" {% if category in selected_categories %}checked="checked"{% endif %} />{{ _(category) }}</label>
{% endfor %}
</div>