Install a theme with its demo content -- via command line?
You can import the database using drush rather than phpMyAdmin.

Create a standard drupal 8 site, go to the /sites/default directory and run 

drush sql-cli &lt; databasefilename.sql


See the importing database section here http://tylerfrankenstein.com/code/use-drush-export-import-drupal-mysql-database-dump-file