Cannot reset theme to plain default

I downloaded a theme which I don't like and I don't see any way to reset to default theme in Directory Opus 12. There is no plain.dlt listed in the available themes. Is there a DO12 compatible plain or default factory theme (not user made) to download?

Themes essentially modify (up to) four pages in Preferences, which can be reset from the Preferences UI itself.

The Plain Theme post has details.