anchor.diff:  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.
  
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: 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.
  
  === Basic Theme
  
  Here's an example of a basic theme file that extends the base theme:
  
  