anchor.diff:  * stabilize font paths in built-in themes by prefixing paths with GEM_FONTS_DIR (#1568)
   * assign page-layout attribute in running content so it can be used select background per layout (#1570)
   * reset numbering of footnotes in each chapter; add chapter xreftext to label of footnote reference that refers to previous chapter (#1639)
  -* align footnotes block to the bottom of the page it is placed on (#1833)
   * add support for assigning the same font file to all font variants (i.e., styles) using the wildcard key (`*`) (#1588)
   * allow caption max-width to be set to percentage of content by passing argument to fit-content function (#1494)
   * allow theme to control text alignment of caption independent of box alignment (#1493)
  
positive.code: * allow theme to control font size of dot leader in TOC
  * allow section to override outlinelevels for self and children using outlinelevels attribute on section
  * use value of author attribute for PDF info and pdfmark if locked by the API (#1778)
  * allow path of ghostscript command to be controlled using `GS` env var (#1791)
  * only extend theme in extends hierarchy once unless modified with !important (#1800)
  * add print-optimized themes (default-for-print and default-for-print-with-fallback-font) (#1699)
  * add support for power operator in theme (with same precedence as multiply and divide) (#1813)
  
negative.code: 
  Compliance::
  
  * drop support for Ruby < 2.5 (which includes JRuby 9.1) (#1681)
  * use YAML.safe_load from Ruby stdlib instead of safe_yaml gem
  * drop deprecated Pdf module alias in API (leaving only PDF)
  * remove deprecated "ascii" fonts; only bundle the more complete "subset" fonts
  