How to delete/customize imported demo content from a theme or add my own?

With your theme you got a docs page with instructions how to edit the pages and use the theme. Don’t delete any php files, you have visual page builder.

If you will navigate into https://your-domain/wp-admin/edit.php?post_type=page you should see the list of pages which were created by importing the demo content.

If you can’t find the docs or not sure how to use it, contact the support.