Sorry to hear this about Strato, normally a Webhost customer service can help you get into your Files. So the problem:
Cant access the Wordpress Admin areaSo we need another way to de-activate/ delete the current Theme. We need access to the Website Files/ Folder.
For an understanding of the process please read this page under #2. Deleting a WordPress theme via FTP (or via ftp manager):
https://www.wpbeginner.com/beginners-guide/how-to-delete-a-wordpress-theme/
I think with Strato its called Server Manager:
https://www.strato-hosting.co.uk/faq/hosting/what-is-ftp-and-how-can-i-use-it/
Please read this, i believe you will need to do this yourself if Strato Customer service will not help you. Let me know your thoughts and talk soon 🌝
Richard