Hi Jonas,
Thanks for posting in the community. We are happy to help you.
I go through your post carefully. Based on my research, I haven't found out-of-box settings or PowerPoint cloud policies that remove/hide the Office built-in template themes.
Regarding your question, since it is related to Intune, and our team focuses on queries about PowerPoint built-in features, to avoid any incorrect information from our side, I would suggest you post a new thread in the Microsoft Q&A Community for queries related to Microsoft Intune. You can post a new thread there (click "Ask a question" and add the tags Office Management, Microsoft Intune Configuration, and Microsoft Intune Application management) to get confirmation and help from experts.
Sorry that our category may have limited resources on checking on your concern further.
Thanks for your understanding.
George | Microsoft Community Moderator