Hi there,
Thanks for writing in.
If it’s just complete new from scratch, then just re-install your whole WordPress site.
If it’s just deleting, then just activate a non-X theme, then delete X theme and child theme on the same page where you’re activating them. Or you can delete them through FTP.
Deleting will not remove demo data that already imported, hence, a fresh install is much more suitable if really wish to start from scratch.
And, you can re-install X theme and child theme the same way you installed them before.
Hope this helps.