Yeah, the green for the username looks out of place. I would just stick to black text and yes, make it bold (not sure if it already is or if it's just a larger font). I like the rest of the text (Joined, Messages, etc) with the size they are. That info isn't used too often so you don't want it jumping out at you, plus, at least for me, and I have pretty poor vision (I'm pretty much blind w/o glasses), are perfectly readable.
The signature font color definitely needs changed. Maybe a darker shade of gray than it is currently. I certainly don't want the signature jumping out or being too prominent, especially given the excessive length of some people's.