hugo-theme-relearn/layouts/partials/custom-header.html

<!-- compat way of adding custom scripts in old Hugo documentation -->
{{- $assetBusting := partialCached "assetbusting.gotmpl" . }}
{{- if or (fileExists "static/css/custom.css") (fileExists "assets/css/custom.css") }}
    <link href="{{"css/custom.css" | relURL}}{{ $assetBusting }}" rel="stylesheet">
{{- end }}
{{- if or (fileExists "static/js/custom.js") (fileExists "assets/js/custom.js") }}
    <script src="{{"js/custom.js" | relURL}}{{ $assetBusting }}" defer></script>
{{- end }}