I agree Sam. I think it would need to be tweaked for very busy and very quiet forums with the default suitable for something in the middle.
For example, on busier boards you might opt to use a font that works better bold (so not as overpowering) or use an icon, or mimic what you have here now.
It needs to be part of the system though, as (and although I didn’t spend much time on it) it wasn’t straight forward to apply the same changes to non-logged in users. (There seems to be a third anonymous view too).
So I’d suggest a style for standard topics, and another for those with unread posts. You could even offer a ‘mark forums read’ button or allow users to set a cut-off rate, such as to only show what’s new since last visit, or as far back as three visits. In the old days, post marking only went back as far as last visit (so only posts made after you logged out (or got logged out) were count as unread). I’m not so keen on this - but setting it to last 3 to 5 visits is worth considering (lower number for busier forums, higher for quieter ones).