anchor.diff:  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).
   
  -Currently, the base theme is always loaded first.
  +Currently, the theme starts out empty.
   Then, the files referenced by the extends key are loaded in order.
   Finally, the keys in the current file are loaded.
   Each time a theme is loaded, the keys are overlaid onto the keys from the previous theme.
  
positive.code: ----
  
  You can also point the extends key at another custom theme to extend from it.
  If you don't want to extend any theme, including the base theme, assign the value `~` to the `extends` key (i.e., `extends: ~`).
  
  WARNING: If you start a new theme from scratch, we strongly recommend defining TrueType fonts and specifying them in the `base` and `literal` categories.
  Otherwise, Asciidoctor PDF will use built-in AFM fonts, which can result in missing functionality and warnings.
  
negative.code: .basic-theme.yml
  [source,yaml]
  ----
  extends: base
  page:
    layout: portrait
    margin: [0.75in, 1in, 0.75in, 1in]
  