Sync Asciidoctor theme with firebird-documentation

src/docs/theme/jaybird-html/README.md

@@ -1,7 +1,14 @@
 firebird-html theme
 ===================
 
-This theme is build using the asciidoctor-stylesheet-factory.
-The source of the theme can be found in branch `firebird` of <https://github.com/mrotteveel/asciidoctor-stylesheet-factory>.
+This theme is a manual modification of the standard Asciidoctor theme (<https://cdn.jsdelivr.net/gh/asciidoctor/[email protected]/data/stylesheets/asciidoctor-default.css>). See also [Customize the default stylesheet](https://docs.asciidoctor.org/asciidoctor/latest/html-backend/default-stylesheet/#customize-the-default-stylesheet).
 
-When updating, remove comments with `/\*.*?\*/` and replace multiple linebreaks using `(?:\r?\n){2,}` replacement `\n`
+Specifically, it includes: 
+
+- different colors (headings and code blocks)
+- some margin changes
+- some tweaks to layout/rendering of some elements
+- extra h7 element (same styling as h6)
+- styles for tocbot (which replaces the default table of contents)
+
+Updating it requires downloading the Asciidoctor default stylesheet and careful diffing (and deciding which changes need to be preserved or not).