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:

https://github.com/asciidoctor/asciidoctor-reveal.js/blob/8f2c82cb2937f2c77f9952f0bb14d76ab6c29301/templates/document.html.slim#L40-L67

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 https://github.com/asciidoctor/asciidoctor-reveal.js/issues/508#issuecomment-1606057615

[refaelsh]

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

Done.