I have tried changing several of the margins and padding to make the widget title bars go all the way across and also make the top ones flush with the header. I can move them around a bit, but they're never quite right.
I've changed this theme quite a bit--making it narrower and less boxy. I wonder if what I'm trying to do is even possible with this theme. There are just so many things in the css that I have a hard time figuring out which one changes what. Some things don't seem to change anything. And I'm a real novice, which I'm sure is obvious.