anchor.diff:  The theme language adds some extra features to YAML, such as variables, basic math, measurements and color values.
   These enhancements will be explained in detail in later sections.
   
  -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._
  +=== Basic Theme
   
   Here's an example of a basic theme file that extends the base theme:
   
  
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:  $ 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.
  