hugo-theme-flat/layouts/partials/header.html

<header class="header-wrapper">
    <div class="header">
        <a class="site-title" href="{{ .Site.BaseURL }}">{{ .Site.Title }}</a>

        <nav class="menu">
            {{ range .Site.Menus.main }}
                <div class="menu-item">
                    {{ if not .Children }}
                        <a href="{{ .URL }}">{{ .Name }}</a>
                    {{ else }}
                        <a>{{ .Name }}↓</a>
                        <nav class="sub-menu">
                            {{ range .Children }}
                            <div class="menu-item"><a href="{{ .URL }}">{{ .Name }}</a></div>
                            {{ end }}
                        </nav>
                    {{ end }}
                </div>
            {{ end }}
        </nav>
    </div>
</header>