The documentation for Asciidoctor PDF has been relocated to docs.asciidoctor.org. The theming guide has been split into two sections, Theming and Theme Keys Reference. Each theming topic and category key has a dedicated page.
On the new documentation site, use the “Search the docs” box in the top navigation bar to help you find the theme key or feature you’re looking for. Also, checkout the What’s New page to learn about the latest Asciidoctor PDF features.