Clarify the Custom Styles documentation page

[refaelsh]

The Custom Styles documentation page is a bit misleading/incomplete:

https://docs.asciidoctor.org/reveal.js-converter/latest/converter/custom-styles/

It states that: "If the :customcss: attribute value is empty then asciidoctor-revealjs.css is the CSS resource that the presentation is linked to."

It does not mention that asciidoctor-revealjs.css must/should be provided by the user.

Here's the actual code:

asciidoctor-reveal.js/templates/document.html.slim

Lines 40 to 67 in 8f2c82c

	link rel="stylesheet" href="#{revealjsdir}/dist/reset.css"
	link rel="stylesheet" href="#{revealjsdir}/dist/reveal.css"
	/ Default theme required even when using custom theme
	link rel='stylesheet' href=(attr :revealjs_customtheme, %(#{revealjsdir}/dist/theme/#{attr 'revealjs_theme', 'black'}.css)) id='theme'
	/! This CSS is generated by the Asciidoctor reveal.js converter to further integrate AsciiDoc's existing semantic with reveal.js
	style type="text/css"
	include asciidoctor-compatibility.css
	- if attr? :icons, 'font'
	/ iconfont-remote is implicitly set by Asciidoctor core. See https://github.com/asciidoctor/asciidoctor.org/issues/361
	- if attr? 'iconfont-remote'
	- if (iconfont_cdn = (attr 'iconfont-cdn'))
	link rel='stylesheet' href=iconfont_cdn
	- else
	/ default icon font is Font Awesome
	- font_awesome_version = (attr 'font-awesome-version', '5.15.1')
	link rel='stylesheet' href=%(#{cdn_base}/font-awesome/#{font_awesome_version}/css/all.min.css)
	link rel='stylesheet' href=%(#{cdn_base}/font-awesome/#{font_awesome_version}/css/v4-shims.min.css)
	- else
	link rel='stylesheet' href=(normalize_web_path %(#{attr 'iconfont-name', 'font-awesome'}.css), (attr 'stylesdir', ''), false)
	= generate_stem(cdn_base)
	- syntax_hl = self.syntax_highlighter
	- if syntax_hl && (syntax_hl.docinfo? :head)
	=syntax_hl.docinfo :head, self, cdn_base_url: cdn_base, linkcss: linkcss, self_closing_tag_slash: '/'
	- if attr? :customcss
	link rel='stylesheet' href=((customcss = attr :customcss).empty? ? 'asciidoctor-revealjs.css' : customcss)
	- unless (_docinfo = docinfo :head, '-revealjs.html').empty?
	=_docinfo

If the :customcss: attribute value is empty then asciidoctor-revealjs.css is the CSS resource that the presentation is linked to.

The documentation is actually true but the extension does not provide it, the user has to provide it. Not sure if it's a good idea to link to a file that might not exists.

It might be worth mentioning that we are also using the following stylesheets regardless of customcss value:

• reset.css (from reveal.js)
• reveal.css (from reveal.js)
• theme/black.css (from reveal.js) - can be overridden using revealjs_theme attribute
• asciidoctor-compatibility.css

Originally posted by @ggrossetie in #508 (comment)

[refaelsh]

Please open a new issue so we can discuss how to improve the documentation.

Done.