Invalid file plain.dlt

So I upgraded my Windows 7 to Windows 10 (I was glad it chose Pro edition from Ultimate).
I discovered that the Directory Opus window was affected by the new look. I thought I would try another theme for Directory Opus at least. The first Theme wasn't accepted by the Lister Theme's Import, so I tried another that worked. Not entirely satisfied with the Theme I wanted to revert to the default/plain look. I did a search and fount out that I needed to import Themes\plain.dlt from the installed Directory Opus location (The directory is "C:\Program Files\System\Directory Opus\Themes" in my case).
My problem is, that it doesn't accept plain.dlt either. It returns the error "Invalid theme file.". I am using Windows 10 with the latest Directory Opus 11. I reinstalled it to get the original look, but still get the same error from importing plain.dlt. It looks OK after the reinstall, but I think it is a bug. I am using Windows 10 Pro 64 bit.

Not sure why we still ship that file. I'll see if we can get it removed as it's obsolete these days.

To revert theme changes back to the defaults:

[ul][li]Open Settings > Preferences.[/li]
[li]Select the relevant page you wish to reset.[/li]
[li]Use File > Reset Page To Defaults to reset it.[/li]
[li]Do any other pages, then click OK.[/li][/ul]

Pages you may want to reset after using themes (depending on what you want to revert):

[ul][li]Display / Colors and Fonts[/li]
[li]Display / Images[/li]
[li]Display / Options[/li]
[li]Miscellaneous / Sounds[/li][/ul]