Search notes:

C:\Program Files\Microsoft Office\root\Document Themes N

C:\Program Files\Microsoft Office\root\Document Themes N (N ∈ 15, 16, …) seems to be related to PowerPoint and stores .thxm files.
This directory as well as its subdirectories is in the default list of MS Office: Trust Center - Trusted Locations for Excel, Word and PowerPoint.

See also

The parent directory C:\Program Files\Microsoft Office\root

Index