Well, I had hopes that the file dialog implementation in Mojave wouldn’t change and you could just keep running Default Folder X 5.2.5 until I finished support for Dark Mode, but Apple had other plans.
Yesterday’s beta release of Mojave changed the file dialogs ever so slightly – just enough for Default Folder X 5.2.5 to say “that looks sorta like a file dialog, but not exactly” and refuse to put up its controls around Open and Save dialogs. So if you’re running the latest Mojave build, you’ll need to visit the Default Folder X beta testing page and download the latest beta build of Default Folder X.
Sorry for the inconvenience, but you know how it is – “moving target” and all that.