anchor.diff:  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`).
  +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.
  +All selector names are implicit (e.g., `heading`), so you customize the theme primarily by manipulating pre-defined property values (e.g., `font_size`).
  +
  +[NOTE]
  +====
  +The theme language in Asciidoctor PDF supports a limited subset of the properties from CSS.
  +Some of these properties have different names from those found in CSS.
  +
  +Instead of separate properties for font weight and font style, the theme language combines these as the `font_style` property (allowing values "normal", "bold", "italic" and "bold_italic").
  +
  +The `text_align` property from CSS is the `align` property in the theme language.
  +
  +The `color` property from CSS is the `font_color` property in the theme language.
  +====
   
   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.
  
positive.code:   font_size_small: $base_font_size * 0.85
  ----
  
  We'll cover more about math expressions in the next section.
  
  === Math expressions & functions
  
  
negative.code: ----
  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
  