anchor.diff:  === Running Content (Header & Footer)
   
   The keys in this category control the arrangement and style of running header and footer content.
  +Please note that the running content will _not_ be used unless a) the periphery (header or footer) is configured and b) the height key for the periphery is assigned a value.
   
   [cols="3,4,5l"]
   |===
  
positive.code:   font_color: #002FA7
  outline_list:
    indent: $base_font_size * 1.5
  ----
  
  When creating a new theme, you only have to define the keys you want to override from the base theme, which is loaded prior to loading your custom theme.
  
negative.code:       content: '\{page-number}'
  |===
  . The background color spans the width of the page, as does the border when a background color is specified.
  . *If the height is not set, the running content at this periphery is disabled.*
  . If the side padding is negative, the content will bleed into the margin of the page.
  . `<side>` can be `recto` (right-hand, odd-numbered pages) or `verso` (left-hand, even-numbered pages).
  Where the page sides fall in relation to the physical or printed page number is controlled using the `pdf-folio-placement` attribute (except when `media=prepress`, which implies `physical`).
  