Install a theme with its demo content -- via command line? The tutorials I've seen all want me to use phpMyAdmin. Here is an example that's very apropos since my theme is Zircon.
https://www.weebpal.com/guides/step-step-guide-install-drupal-8-theme
AFAIK I don't have MyPHPAdmin and I don't have access to it. What I do have is sudo access on the Ubuntu 16.04 server.
I'm using Drupal 8
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