It's not nice, trust me. Since the posts are all left justified by default and then having the boxes centred it looks odd and annoying. Especially since there are many different sizes of screens its just not a good time. I also tried it with a fluid width for the boxes and it still looked messed up being centred because the text for each line is generally fairly short so you are left with this wide box that is half empty.
Theme issue, I've tried to fix it and haven't been able to so far.
I might be able to do this (second one) but don't count on it as I would have to do it manually for every single theme...
There is a feature for this in vB4 but it was buggy when I tested it out while setting up the new forums and haven't gotten around to tweaking it to make it more reliable. There have been more important issues needing attention.