You could use the GeneratePress child theme (which is a blank theme) and build the client a “custom” theme from there. I do this all the time for clients.
Give them a “custom” theme by changing the child theme’s style.css
/*
Theme Name: GeneratePress Child
Theme URI: https://generatepress.com
Description: Default GeneratePress child theme
Author: Tom Usborne
Author URI: https://tomusborne.com
Template: generatepress
Version: 0.1
*/
to this:
/*
Theme Name: My Client Company
Theme URI: https://clientdomain.com
Description: Custom Theme for Client Name
Author: Your name
Author URI: your domain
Template: generatepress
Version: 2.1
*/
When they login to WordPress the Theme Name will be displayed as whatever you enter for Theme Name.