When you install a theme Drupal tries to build a block layout automatically, but the result only works for front-end themes. For admin themes you need to provide a default block layout. See the default config of the base theme:
core/themes/claro/config/optional/
block.block.claro_breadcrumbs.yml
block.block.claro_content.yml
block.block.claro_help.yml
block.block.claro_local_actions.yml
block.block.claro_messages.yml
block.block.claro_page_title.yml
block.block.claro_primary_local_tasks.yml
block.block.claro_secondary_local_tasks.yml
You can copy these files, adjust the file names and replace the theme name within the file. If you copy the files from an exported config then remove UUID and config hash.
If you want to install this only a few times you can also visit the block layout and move/create blocks until it looks the same as the layout of the installed base theme. After that you can always copy the result of the exported config to the themes folder to make further installations easier.