That theme assigned images to specific named folder formats and toolbars.
That's no longer supported in Opus 13, as it never really worked properly. (Everyone has potentially different toolbars and folder formats.) (Also because it wouldn't play well with switching between Light and Dark mode.)
You can fix things up by going to Preferences / Colors and Fonts / Images and assigning the background image to the elements there. (The images there are included in themes, so you could re-save the theme afterwards to get one that works better with the new version.)
The background image from the theme will already be in your Images folder if you have loaded the theme; it just needs assigning to the elements you want it on.