Here's how I archived all the theme files after grabbing the HTML pages with my usual wget-mirror alias:

  [lammy@popola#moz_addons] pwd
  /spinthedisc/Backups/WWW/Computers/Mozilla/Appearance/projectit.com/moz_addons

  [lammy@popola#moz_addons] grep moz_addons ../*.html | \
  sed -E "s/.*moz_addons\/([^\"'\\]*).*/https:\/\/projectit\.com\/moz_addons\/\1/g" | \
  uniq | xargs wget