Issue #2193691 by holly.ross.drupal, Bojhan, BLadwin | mgifford: New methods,...

Issue #2193691 by holly.ross.drupal, Bojhan, BLadwin | mgifford: New methods, standards for describing Seven themes admin page.

core/themes/seven/seven.info.yml

 name: Seven
 type: theme
-description: 'A clean and friendly administration theme.'
+description: 'The default administration theme for Drupal 8 was designed with clean lines, simple blocks, and sans-serif font to emphasize the tools and tasks at hand.'
+alt text: 'Default administration theme for Drupal 8 with simple blocks and clean lines.'
+tags: 'multi-column, fluid, responsive, sans-serif, accessible'
 package: Core
 version: VERSION
 core: 8.x