system.theme.global.yml

system_theme_settings in drupal/core/modules/system/system.admin.inc

Form builder; display theme configuration for entire site and individual themes.

system_update_8055 in drupal/core/modules/system/system.install

Move system theme settings from variables to config.

favicon:
  mimetype: image/vnd.microsoft.icon
  path: ''
  url: ''
  use_default: '1'
features:
  comment_user_picture: '1'
  comment_user_verification: '1'
  favicon: '1'
  logo: '1'
  name: '1'
  node_user_picture: '1'
  main_menu: '1'
  secondary_menu: '1'
  slogan: '1'
logo:
  path: ''
  url: ''
  use_default: '1'