anchor.diff:   $ ASCIIDOCTOR_PDF_DIR=`gem contents asciidoctor-pdf --show-install-dir`;\
      cp "$ASCIIDOCTOR_PDF_DIR/data/themes/default-theme.yml" custom-theme.yml
   ====
  +
  +=== Key Nesting
   
   Keys may be nested to an arbitrary depth to eliminate redundant prefixes (an approach inspired by SASS).
   Once the theme is loaded, all keys are flattened into a single map of qualified keys.
  
positive.code: All the available keys are documented in <<Keys>>.
  The converter uses the information from the theme map to help construct the PDF.
  
  Instead of designing a theme from scratch, you can extend the default theme using the `extends` key as follows:
  
  [source,yaml]
  
negative.code: Therefore, if you have a background in web design, the terminology should be immediately familiar to you.
  *Note, however, that the theming system isn't actually CSS.*
  
  The theme file must be named _<name>-theme.yml_, where `<name>` is the name of the theme.
  _We recommend *not* using the names *base* or *default* so you don't confuse it with one of the built-in themes._
  
  === Selectors and Properties
  
  Like CSS, themes have both selectors and properties.
  Selectors are the component you want to style.
  The properties are the style elements of that component that can be styled.
  