anchor.diff:  ----
   base:
     font_color: #ff0000
  +----
  +
  +It's also possible to specify no color by assigning the special value `transparent` as shown here:
  +
  +[source,yaml]
  +----
  +base:
  +  background_color: transparent
   ----
   
   ==== RGB
  
positive.code: The theme language in Asciidoctor PDF is based on the http://en.wikipedia.org/wiki/YAML[YAML] data format and incorporates many concepts from CSS and SASS.
  Therefore, if you have a background in web design, the theme language should be immediately familiar to you.
  
  Like CSS, themes have both selectors and properties, but only a fraction of what CSS supports.
  Unlike CSS, all selectors are implicit (e.g., `heading`), so you customize the theme primarily by manipulating pre-defined property values (e.g., `font_color`).
  
  A theme (or style) is described in a YAML-based data format and stored in a dedicated theme file.
  YAML is a human-friendly data format that resembles CSS and helps to describe the theme.
  
negative.code: The theme language supports color values in three formats:
  
  Hex:: A string of 3 or 6 characters with an optional leading `#`.
  +
  The special value `transparent` indicates that a color should not be used.
  RGB:: An array of numeric values ranging from 0 to 255.
  CMYK:: An array of numeric values ranging from 0 to 1 or from 0% to 100%.
  
  