Seven Sub-theme shows Branding and Nav instead of Local Tasks / Admin Menu [8]
I think you need to copy the regions as well.  Here's the YML from our custom Admin theme.

name: MY Admin Theme
type: theme
description: 'My administration theme'
core: 8.x
base theme: seven
libraries:
  - my_admin/global-styling

regions:
  header: 'Header'
  pre_content: 'Pre-content'
  breadcrumb: Breadcrumb
  highlighted: Highlighted
  help: Help
  content: Content
  page_top: 'Page top'
  page_bottom: 'Page bottom'
  sidebar_first: 'First sidebar'

ckeditor_stylesheets:
  - ../main_theme/css/style.css