ABOUT SEVEN
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Seven is the default administration theme for core in Drupal 8. This theme is
used on all admin pages and by default on node edit forms.
To read more about the Seven theme's origins (in Drupal 7) please see:
https://www.drupal.org/docs/7/core/themes/seven
See https://www.drupal.org/docs/8/core/themes/seven-theme for more information
on using the Seven theme.
Seven is an internal theme and shouldn't be extended by other themes. Please
see https://www.drupal.org/node/2582945 for more info.
ABOUT DRUPAL THEMING
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See https://www.drupal.org/docs/8/theming for more information on Drupal 8
theming.