redstrate.com/themes/red/layouts/partials/sidebar.html

<nav>
    <details id="nav-menu" open>
        <summary>Navigation</summary>
        {{ $icon := resources.Get "external-link.svg" }}

        {{ range .Site.Menus.main }}
            <div class="site-menu">
                <a href="{{ .URL  }}">{{ .Name }}{{ if strings.HasPrefix .URL "http" }}<img class="external-link" aria-hidden="true" alt="External link" width="16" height="16" src="{{ $icon.Permalink }}"/>{{- end -}}</a>
            </div>
        {{ end }}
    </details>
</nav>