I know there have been about 1000 posts of people asking about a new theme, and I know phpBB theme development and design can take a long time (source: my own failed attempts) and the admins here put as much time as they can into this forum, but they all have lives so I'm not asking for that... but reading this forum late at night as I do sometimes, now is an example, writing this at 1 AM I find myself using invert colors to reduce the bright white light. It's not a huge deal, but I think it would be a nice, and knock on wood... easy addition to the forum.
I after a little bit of searching found a few options for theme switching and a somewhat similar to default dark theme that could be used in the short (or long) term.
I'd be happy to send over the links, but depending on the phpBB version the site is running they may be incompatible.
I don't sleep well this time of year don't judge.