Configuration THEMENAME.settings depends on the THEMENAME theme that will not be installed after import.

This came up when trying to migrate a full config export from a test site to live.
However, exporting config from test and then trying to re-import to test caused the same issue.

The site has the latest Bootstrap theme - https://www.drupal.org/project/bootstrap → - using the LESS starterkit.

The following steps meant the config could be imported on the live site.

1. exported entire site config from dev site with new feaures in it
2. clobbered dev database with a sync from live
3. manually deleted all staged config on dev from sites/default/files/config_xxxxxx
4. manually deleted all staged config on live from sites/default/files/config_xxxxxx
5. opened exported config file (.tar.gz) in item 1 and deleted the THEMENAME.settings.yml file
6. imported site config into dev site & tested
7. imported site config into live
8. All good

There is a short forum discussion on this at https://www.drupal.org/node/2698777 → and that's the only result on Google at the moment.

Unsure if this is a core bug, a bootstrap bug or an error somewhere in our development.