hugo-book-theme/layouts/partials/docs/html-head.html

<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>{{- template "title" . }} | {{ .Site.Title -}}</title>

<!-- Fonts for the theme -->
<link href="https://fonts.googleapis.com/css?family=Oxygen|Oxygen+Mono:300,400,700" rel="stylesheet">

<!-- Theme stylesheet, if possible do not edit this stylesheet -->
<link rel="stylesheet" href="{{ "normalize.min.css" | relURL }}">
{{ $styles := resources.Get "book.scss" | resources.ToCSS | resources.Minify | resources.Fingerprint }}
<link rel="stylesheet" href="{{ $styles.RelPermalink }}">

<!-- Custom stylesheet - for your changes -->
{{ if (fileExists "styles.css" | relURL) -}}
<link rel="stylesheet"  href="{{ "styles.css" | relURL }}?v={{ now.Unix }}" type='text/css' media='all'>
{{- end }}

<!-- Favicon -->
<link rel="icon" href="{{ "img/favicon.ico" | relURL }}" type="image/x-icon">

<!-- RSS -->
{{ if .RSSLink }}
<link href="{{ .RSSLink }}" rel="alternate" type="application/rss+xml" title="{{ .Site.Title | default "" }}" />
<link href="{{ .RSSLink }}" rel="feed" type="application/rss+xml" title="{{ .Site.Title | default "" }}" />
{{ end }}

{{ "<!--" | safeHTML }}
Made with Book Theme
https://github.com/alex-shpak/hugo-book
{{ "-->" | safeHTML }}