Lead Developer
Definitely a big site, but I don’t believe Duplicator does anything to your live site, it just creates a package which you can use to deploy a copy of your site somewhere else.
There’s other options out there too – BackupBuddy etc.. If you search for “WordPress copy site” or something along those lines you’ll find some alternatives.
Either way, you should be taking database and file backups of your current site at least once a week.
Documentation: http://docs.generatepress.com/
Adding CSS: http://docs.generatepress.com/article/adding-css/
Ongoing Development: https://generatepress.com/ongoing-development