anchor.diff:  === Base
   
   The keys in this category provide generic theme settings and are often referenced throughout the theme file as variables.
  -It's common to define additional keys in this category (e.g., `base_border_radius`) that serve as custom variables to help keep your theme DRY.
  +We recommended that you define this category after the page category and before all other categories.
  +
  +NOTE: While it's common to define additional keys in this category (e.g., `base_border_radius`) to keep your theme DRY, we recommend using <<Custom Variables,custom variables>> instead.
   
   [cols="3,4,5l"]
   |===
  
positive.code: === Variables
  
  To save you from having to type the same value in your theme over and over, or to allow you to base one value on another, the theme language supports variables.
  Variables consist of the key name preceded by a dollar (`$`) (e.g., `$base_font_size`).
  Any qualified key that has already been defined can be referenced in the value of another key.
  (In order words, as soon as the key is assigned, it's available to be used as a variable).
  
  For example, once the following line is processed,
  
  
negative.code: == Keys
  
  This section lists all the keys that are available when creating a custom theme.
  The keys are organized by category.
  Each category represents a common prefix under which the keys are typically nested.
  
  TIP: Keys can be nested wherever an underscore (`_`) appears in the name.
  This nested structure is for organizational purposes only.
  All keys are flatted when the theme is loaded (e.g., `align` nested under `base` becomes `base_align`).
  
  The converter uses the values of these keys to control how most elements are arranged and styled in the PDF.
  The default values listed in this section get inherited from the https://github.com/asciidoctor/asciidoctor-pdf/blob/master/data/themes/base-theme.yml[base theme].
  
  IMPORTANT: The https://github.com/asciidoctor/asciidoctor-pdf/blob/master/data/themes/default-theme.yml[default theme] has a different set of values which are not shown in this guide.
  
  When creating a theme, all keys are optional.
  Required keys are provided by the base theme.
  Therefore, you only have to declare keys that you want to override.
  
  [#keys-page]
  === Page
  
  The keys in this category control the size, margins and background of each page (i.e., canvas).
  We recommended that you define this category before all other categories.
  
  NOTE: The background of the title page can be styled independently.
  See <<Title Page>> for details.
  