I was a light old guy all the way until the site auto swapped today. Something about light new doesn't work for me for some reason. I think the lack of borders gives a too zoomed in look for me or something on mobile, much prefer dark new instead.
As far as changes, I would say I would enjoy the font being a brighter white on topics that have unread/new posts on the new dark theme. To me, the color on the main board screen somewhat blends in with topics where I've already read the latest posts. In some fashion, have a more distinct separation between read/unread in that dark new theme.