Eh...thinking on this, I think that this particular theme (ie Darcula) isn't really suited for a Dark Mode. The contrast is a little too harsh (for example, the reds on MekHQ's Personnel tab look REALLY bright, which kinda hurts my eyes and makes it uncomfortable to read, plus the reds on MegaMek don't really show up too well when I read a "Unit Destroyed" message) and certain options seem to blend together (such as when I go to a drop-down menu or a right-click menu and try to click one option, say editing statistics, but wind up clicking another option instead; this may have to do with how certain options are spaced with each other).
The theme of Flat Dark is also ill-suited for a dark mode option and suffers the same issues as Darcula. I say this as someone who primarily uses dark mode for a lot of (mobile) applications, but I firmly believe that a dark mode for MekHQ/MegaMek/MekLab would need to address the issues I raised before I feel comfortable using it. Maybe see if there's a UI/Accessibility person out there who's willing to help with this?
For transparency's sake, I voted "yes" for making dark mode default--however, y'all have to at least do something about the contrast issues, certainly, before I consider moving on from CDE/Motif as my "impromptu dark mode."