anchor.diff:  
   ////
   Topics remaining to document:
  -* document which attributes can be set in document (pdf-page-size, front-cover-image, back-cover-image, etc)
  +* document which attributes can be set in document (pdf-page-size, front-cover-image, back-cover-image, etc) (issue #428)
   * line height and line height length (and what that all means)
   * title page layout / title page images (logo & background)
   * document that unicode escape sequences can be used inside double-quoted strings
   ////
   
  -The theming system in Asciidoctor PDF is used to control the layout and styling of the PDF file that Asciidoctor PDF generates from AsciiDoc.
  -The theme is driven by a YAML-based configuration file.
  -This document explains how the theming system works, how to define a custom theme and how to enable the theme when running Asciidoctor PDF.
  -Please note if you are using a custom theme, you are expected to override some of these sections below (in particular fonts, otherwise PDF fonts will be used).
  +[.lead]
  +The theming system in Asciidoctor PDF is used to control the layout and styling of the PDF file Asciidoctor PDF generates from AsciiDoc.
  +This document describes how the theming system works, how to define a custom theme in YAML and how to activate the theme when running Asciidoctor PDF.
  +
  +IMPORTANT: If you're using a custom theme, you're expected to override some of keys described in the sections below, in particular fonts.
  +If you don't define your own fonts, the built-in PDF fonts will be used.
   
   toc::[]
   
  
positive.code: 
  You can split the content value across multiple lines using YAML's multiline string syntax.
  In this case, the single quotes around the string are not necessary.
  To force a hard line break in the output, add [x-]+{space}{plus}+ to the end of the line in normal AsciiDoc fashion.
  
  [source,yaml]
  ----
  
negative.code: 
  TIP: The running header and footer can be disabled from the document by setting the `noheader` and the `nofooter` attributes, respectively, in the document header.
  
  NOTE: If content is not specified for the running footer, the page number (i.e., `\{page-number}`) will be shown on the left side on verso pages and the right side on recto pages.
  
  NOTE: The background color spans the width of the page.
  When a background color is specified, the border also spans the width of the page.
  