Apple Music Preview
Tons of tons of websites tell you how to enable dark mode in iTunes. Those sites are clickbait. However, this program, also made by Apple, behaves a lot like iTunes and it has dark mode.
You can import files and organize our library just the same. It is basically the new version of iTunes, which is why the actual program named iTunes is not being updated to include dark mode. The only downside of Apple Music Preview compared to iTunes is that everytime you search your own library, you have to toggle to search your library instead of the iTunes music store. I find this tradeoff worth it to be able to use dark mode to view my iTunes music collection.