Why do I have error related to unknown themes during synchronization?
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.