By Chat Topic I mean the "Entering the Rift" line that pops up when you enter chat. I notice it isn't working properly and thougjt I might make myself useful and explain how to fix it, in the event that you don't already know and just haven't gotten around to it.
You probably know what system messages are, but if you don't you can access it via Special:AllMessages.
The specific page we're looking for is MediaWiki:Chat-welcome-message. For whatever reason, the text displays fine here, but not in chat. Usually you can fix this by closing the tag after the message. In this case, however it wouldn't work. From what I can tell, it's because it doesn't recognize the Font-face tag. Remove that tag and it might go back to working properly.
To the specific font you want, font-face doesn't work, so to fix this: append ( font-family: impact ) after the last semicolon in the div tag. Speaking of which I notice there's more than one. You may want to combine them in order to prevent further conflicts.
Hopefully it works, but if it doesn't I'll get back to you with more info.