Rework stylesheets to use the new Asciidoctor approach

[mrotteveel]

The current HTML theme/stylesheet was created using a modified variant built with asciidoctor-stylesheet-factory (specifically this fork, branch firebird). However, that is no longer how the Asciidoctor stylesheet is maintained (AFAIK), so might now be lagging behind in stylesheet changes.

To this end, the stylesheet should be rebuilt and reworked to be built as detailed in Customize the default stylesheet.