It's not really a bug, tho -- it's just that new features were added, so the styles had to be altered to accommodate them. Technically, I assume, you should only have to alter the new parts which are:
.newtotal, is specified under Category/Forum Properties and is used to show the number of new posts/topics.
.popup_content, is specified under Misc. Properties and is used for the popups on the Active Topic screen and a couple other areas.
.post-buttons, is specified under Post Properties and is used for the various options when viewing a post, such as reply, quote, etc. We're no longer using just images for these since we've reintroduced multiple languages.
You can install a 7.1 style sheet as a new sheet, then look into it to see the new properties that've been added. It shouldn't take more than a few minutes I would think, working with them side-by-side...
(Of course, this isn't counting that problem you're having with the column names in Active Topics - tho as you said it doesn't appear to be a Styles issue)
