I can take a look at it, no promises. I like sticking with the default theme because it makes the forum easy to upgrade when new versions of phpBB are available.
If I add a custom dark theme then I have to worry about what's going to break with that theme when upgrading to a new version of phpBB.
You're also at the mercy of the theme creator to update their theme to work with the new version of phpBB.
Since forums are often targets for malicious attacks, I tend to always want to stay at the latest software, so I wouldn't want to hold back an upgrade due to a theme not being compatible with the forum. Of course, I could just force everyone to use the default theme after an upgrade until all themes are patched to work with the new software.
I wouldn't mind a dark style for phpBB.
Not only do these bright colors make me feel dizzy after awhile, but colorized text is limited due to blending.
I'm sure that if an upgrade involved scrapping a style, we could live without a dark setup until a suitable replacement is found.
Going by their repo, I doubt we'd have to deal with broken styles often. However, I would like to stress how I severely dislike third-party plugins. It appears that phpBB styles utilize more than just color data. I would be more concerned with the file contents than anything else.