I'm not sure where to ask this. On this site if you show the active topics from the last 24 hours and you move the cursor over a link it show part of the contents of the post against an opaque background.

In my test installation at http://test.twoplustwo.com hovering shows the text against a transparent background making it unreadable.

I imagine this is a style sheet issue, but it isn't obvious to me what needs to be fixed and how.

This is a new stylesheet addition in 7.1B4 I believe; just use the style editor and edit the following class:
CP -> Display Options -> Styles -> Edit Theme -> Misc. Properties -> .popup_content

An example of a class for it:

Thanks. Was that documented somewhere? I thought we'd updated our styles but perhaps we missed some others?

This post tells you about the new style classes added with 7.1b4+

The beta announcement for the public build announced the 3 new classes; the 7.1 release announcement when its final will as well (plus the newsletter when Rick toots his own horn hehe)