This is a bit obscure, but is meant as a hint for others trying to find a fix.
Starting sometime in the near but foggy past, my OO docs started showin thumbnails. I would much prefer the old bright orange icon. It makes it far easier to find OO files in a list.
After mucking around, I finally found that by going to System Preferences > Extensions > Quick Look, I could uncheck the box for OmniOutlinerThumbnails. This returned icons to their old high-contrast orange color.
I don’t see the point in seeing the thumbnail for a document, so (in my opinion), the thumbnails a feature that should be unchecked by default.
Thumbnails for photos, movies, graphic design docs (see: OmniGraffle), or other intrinsically visual things is useful. Thumbnails for textual things is kinda pointless, as they all look about the same. Just imagine picking a book off the shelf based on what the text in the book looks like