Yes, you can!
Finding a good dark theme for XenForo is notoriously difficult. A lot of users love complaining about a dark theme here. I've spent a lot of my personal time trying to find a good, reliable theme that is pleasant to use. There are no dark themes (free or paid) for XenForo that fit Otland. I'm sure if XenForo had an official one (and I think it will in the future eventually), we'd have that. But it's not a priority for XenForo and therefore we don't have a perfect one yet.
Now, how can you bring it back? Some users actually did more than just complaining here on the forum and blaming me for being lazy. They actually helped by taking an "okay" dark theme and working on it to make it better. That, of course, didn't go unpunished by some here in the Feedback section complaining about it still. As I've always told those who offered help, I am willing to install a dark theme that meets our criteria. So yes, you can bring a dark theme back if you're willing to do the work.
Now, what happened to the dark themes we had?
First, they were unmaintained by the original upstream maintainers. I'm sure most of you can't see that but we upgrade the forum very often, almost always instantly when a stable release comes out. The fact that you can't see it is a testament that it's been done correctly - invisible to the user. The Staff helps test new releases on a testing copy of the forum, etc. A theme without updates is lagging behind XenForo and some parts of it look out of place.
As numerous users also pointed out, there were some problems with those themes from day 1. Most common were around code highlighting and templates added by some addons. Also, you may not know that our default light theme also carries custom modifications. One of these is the visual separation of sticky threads, which, as
@Blackstone pointed out, is not a feature of most dark themes.
Every such additional theme takes work to integrate all custom modifications, which with the frequency of the updates and lack of upstream maintenance required me to drop some of the obsolete dark themes. Instead, I've uploaded two new options as a current replacement - Shades of Blue and Shades of Grey. You have to opt into them manually but they are available.
@Okke even noticed a new experimental theme that I uploaded because it was just recently released (after this feedback thread was opened) and I agree that it's horrible, therefore it was removed.
You can keep complaining but there won't be a perfect dark theme unless someone creates one. It currently does not exist. I will try to keep at least one darker-looking theme but if above mentioned circumstances arise again, existing dark themes may be removed and replace with something different - or not.