If there are no announcement topics, the container for topiclist get the classes: "forumbg announcement".
The code involved is line 126 in viewforum_body.html (Prosilver):
If there's at least one announcement topic the code works, and only the container div for the list of announcement topic(s) get the class "announcement" appended, and the list of announcement topics is separated from the rest of the topics.
If there are no announcement topcis the only container div for topiclist will still be appended with the class "announcements" in addition to the "forumbg" class.
Hope I made it clear enough to understand.
I talked to rxu on the irc channel about it as well, and was told to create a ticket here.