Why do I have error related to unknown themes during synchronization? When I try do synchronize from my local site to the remote one, I get this bunch of errors which prevent configuration to be imported.  



It seems to be related to some themes (alpha_responsive_theme, elegant_theme, mm, nucleus, tb_sirate, zeropoint) but why?
I'm pretty sure I don't have these theme in my configuration: I have these folders in core/themes bartik, classy, engines, seven, stable, stark and onepage_zymphonies_theme which is set to be the default one (seven being the Administration theme).  

Any idea why it happens?
And how to get rid of it?
Thanks in advance.
At some point, the config you are using has had those themes enabled.  Either from another site that you got the config set/ database from (e.g. the distro install?) or the themes were install/uninstalled/deleted on the dev machine (testing different themes?).  

Since you seem to have the *.settings files for the themes, these were probably installed and then deleted without uninstalling them.  When a theme is uninstalled, this configuration information is removed.

To get rid of them, add back in the themes, re-enable them and then uninstall them.

An alternative is to use "drush cli"  / the log messages to identify the configuration object names.  Then use "drush cdel" to remove them from your dev box.  NOTE: backup your db prior to this!

Or you can find the yaml files in the exported configuration and remove them.  But you will have to do this each time you export to the remote site.

Another possible way is to try the easy uninstall project. Haven't used it but is seems to provide a GUI way to clean up the configuration.

Soapbox Warning 

IMHO this is one of the great problems with Drupal 8 using a massive blob of config files.  Developers will test stuff and decide not use it.. and it's impossible to find the cruft in the massive blob... 

Features is now available for D8.  It's a pain to set up, but your large blog can become a more manageable set of smaller config sets.