drupal 8 cannot import config themes will be disabled
It's always the paupers shoes that are the worst right? In much the same spirit, I updated this very website in a very early version of D8 to prove that I could, along with familiarize myself a bit more with Drupal 8. I ran into many issues, as I migrated from D6 to D8.
A new issue relative to this arrived when I finally got around to exporting my configuration to code and using it to deploy configuration changes from local to live.
When running my configuration import, I got the following error output:
The import failed due for the following reasons: [error] Configuration <em class="placeholder">block.block.menu</em> depends on the <em class="placeholder">layout6-09</em> theme that will not be installed after import.
Essentially, what I was seeing was that I had block configurations in my database, pointing to themes that don't exist. This never is an issue, even during export. It's only an issue with you attempting to run an IMPORT because then the checks are performed.
The simplest solution to all of this:
Find the blocks in question in your configuration directory. The block files will have the same name as mentioned above, such as block.block.menu.yml. Remove the files called out, verifying of course that the configuration file does in fact reference old themes. My example above referenced a Drupal 6 specific theme I had written, but often issues relative to Garland will remain (a theme in D6 gone in D8) that were ported over from D6 for some reason.