Unknown theme error

Problem/Motivation

After enabling the designs_layout module and using the adminimal theme as my admin theme, when I flush caches on an admin page, I get the following:

Drupal\Core\Extension\Exception\UnknownExtensionException: The theme Seven does not exist.

The code is trying to load the base theme by its name instead of its machine name.

Steps to reproduce

- Enable designs_layout
- Enable theme with a base theme
- Flush caches and reload a page

Proposed resolution

Adjust DesignManager:392 to

foreach (array_keys($themes) as $theme) {