Imo Xenforo with whatever default or even somewhat customized theme would change the board's vibe, if just that there's a certain raw quality about older board themes that suits the posts, unlike many boards' Fisher-Price'd themes that have migrated to other software.
If the same theme was recreated it'd be fine (though unlikely) but at the same time making the site more mobile friendly and adding a dark would only require stylesheet additions and changes anyway, it's just no one has yet spent the time to.