Moved this to a new thread.
Actually it is something every forum admin has to consider.
With one theme it's a lot easier to make changes and keep up with coding, plus you know you have uniformity and you know how everyone sees your pages. It's easier to keep your image or "branding" the way you want it as well.
With a choice of several themes you have the added benefit of selection in the case that some users do not like your default theme.
I personally love the dark theme and think it is easier on the eyes and easier to read. To me the white theme is just too bright and it's hard for me to read anything on it. I know some users would say the opposite is true for them. It's nice to be able to please everyone if possible.
At this point I am trying to maintain two themes, but it gets tricky sometimes and it certainly does create a lot of extra work. What usually happens is the dark theme which is set to the forum default gets all the new features, and it takes a while before the light theme gets stuff added. It's just the cost o having more options.