It is possible, the real question is how much effort would it take. What does the Flarum database structure look like?
> Would it be better to just start from zero, considering the forum is barely a month old?
If the community is willing to restart that would obviously remove any effort of porting users/threads/posts, but some members may be annoyed by that - nobody here has enough info to say which is preferable.
The database structure appears to be described here - a converter would involve mapping each table/field of Flarum to an equivalent in Beehive then doing a bunch of SQL select/insert statements in the right order - it's not necessarily difficult (for someone knowing SQL), but it could be time-consuming.
Beehive has themes, which means anyone who knows CSS can change the colours to whatever they like.
Creating a "dark mode" would probably involve starting with the "Black" theme (which is more a mix of greys) and updating all the bits of it which are still black text and white backgrounds.
Others might not see some of those as issues, but it's their combination along with many other individually minor things which collectively say "this is shit".
> I'm asking because I am trying to figure out why it would have been selected in the first place, other than it is free.
People are attracted to things that are familiar, so the more people have used similar "modern" software in the past, the more crap like Flarum will appeal to them.
> BTW, the Wikipedia page is way out of date. I don't know if forums will make a comeback, but if they do - Beehive should be the first and last stop.
A double-edged benefit/drawback of Wikipedia is that they don't like people too connected to a subject to write about it - in some ways we should be happy that the Beehive article still exists at all; a lot of other Open Source software articles I previously tried to create haven't survived.
To get a better article (and more attention/use in general) would need reviews in credible tech websites, and they generally need a reason to write about a topic in the first place.
"Active development" is also a way of saying "incomplete" and "subject to change". Too much weight/credit is given to stuff which is new and unproven.