anchor.diff:  - default
   - ./brand-theme.yml
   |===
  +
  +=== Extending the Font Catalog
  +
  +If you define a <<Custom fonts,font catalog>> in a theme that extends from `default`, and you want to continue to use the bundled fonts in your theme, you either have to redeclare the bundled fonts:
  +
  +.Redeclaring the bundle fonts in a custom theme
  +[source,yaml]
  +----
  +extends: default
  +font:
  +  catalog:
  +    Noto Serif:
  +      normal: GEM_FONTS_DIR/notoserif-regular-subset.ttf
  +      bold: GEM_FONTS_DIR/notoserif-bold-subset.ttf
  +      italic: GEM_FONTS_DIR/notoserif-italic-subset.ttf
  +      bold_italic: GEM_FONTS_DIR/notoserif-bold_italic-subset.ttf
  +    M+ 1mn:
  +      normal: GEM_FONTS_DIR/mplus1mn-regular-subset.ttf
  +      bold: GEM_FONTS_DIR/mplus1mn-bold-subset.ttf
  +      italic: GEM_FONTS_DIR/mplus1mn-italic-subset.ttf
  +      bold_italic: GEM_FONTS_DIR/mplus1mn-bold_italic-subset.ttf
  +    Your Font:
  +      normal: /path/to/your/font.ttf
  +heading:
  +  font-family: Your Font
  +----
  +
  +or you need to set `merge: true` above your font definitions:
  +
  +.Merging with the inherited font catalog
  +[source,yaml]
  +----
  +extends: default
  +font:
  +  catalog:
  +    merge: true
  +    Your Font:
  +      normal: /path/to/your/font.ttf
  +heading:
  +  font-family: Your Font
  +----
  +
  +If you're referring to a bundled font, you'll need to prefix the path with `GEM_FONTS_DIR` (or add it to the value of the `pdf-fontsdir` attribute) so the converter can find and register it.
  +You can find the bundle font definitions in default theme.
   
   [#keys-role]
   === Role
  
positive.code: If the filename begins with `./`, it's resolved as a theme file relative to the current theme file.
  Otherwise, the filename is resolved as a theme file in the normal way (relative to the value of the `pdf-themesdir` attribute).
  
  CAUTION: If you define the <<Custom fonts,font catalog>> in a theme that extends from `default`, you either have to redeclare any built-in font that on which the combined theme depends, or you need to set `merge: true` above your font definitions.
  You can find the built-in definitions in default theme.
  If you are referring to a bundled font, you'll need to prefix the path with `GEM_FONTS_DIR` or add it to the value of the `pdf-fontsdir` attribute so the converter can find and register them.
  
  Currently, the base theme is always loaded first.
  Then, the files referenced by the extends key are loaded in order.
  Finally, the keys in the current file are loaded.
  
negative.code: Topics remaining to document:
  * line height and line height length (and what that all means)
  * title page layout / title page images (logo & background)
  ////
  
  [.lead]
  The theming system in Asciidoctor PDF is used to control the layout and styling of the PDF file Asciidoctor PDF generates from AsciiDoc.
  This document describes how the theming system works, how to define a custom theme in YAML, and how to activate the theme when running Asciidoctor PDF.
  
  TIP: The quickest way to create your own theme is to <<Extends,extend the default theme>>.
  This gives you a set of foundation styles to build on and provides a collection of <<Bundled Fonts,bundled fonts>> (which you don't have to redeclare).
  If you want to use your own fonts, you must declare the name and location of each font in the <<Custom Fonts,font catalog>>.
  If you want to combine your own fonts with the bundled fonts, you can either extend the font catalog (by setting `merge: true`) or <<Extending the Font Catalog,redeclare the bundled fonts>> alongside your own.
  
  WARNING: If the theme doesn't specify any fonts (nor are they being inherited), only the built-in (AFM) fonts provided by the PDF reader will be available.
  Using AFM fonts can result in missing functionality and warnings.
  See the <<Built-In (AFM) Fonts>> section to learn more about these limitations.
  
  