hugo-theme-flat/layouts/partials/side-categories.html

<div class="side-categories">
    <h2>Categories</h2>
    <hr />

    <ul>
        {{ range $key, $_ := .Site.Taxonomies.categories }}
            <li>
                <a href="{{ "categories/" | relURL }}{{ $key | urlize }}">{{ $key }}({{ .Count }})</a>
            </li>
        {{ end }}
    </ul>
</div>