Well a few things came to my mind:
1: It's quite hard to fit "No, you have already viewed every topic in this forum at least once and there are no new posts nor threads in this forum or any one of it's possible subforums." inside that little box
2: It contrasts much more when there isn't a word in there. This way, it's far easier to tell if there's a new post or not. Having the word "Old" is still three letters, so it's hard to tell the difference with just a quick skim or glance - colors, on the other hand, are easy to tell apart.
3: I have absolutely no idea of what font style (face/family, size, color, weight, etc) was used. I'd have to guess. This is what I did while I was making the Chat buttons for each style. As you can see, in the Visual Depth theme, the Chat button looks a little off. I tried to make them look a like as much as possible, but it's quite hard with so many options.