I don't like anything about that menu, but a quick-and-dirty fix is basically that your submenu (the .menu ul) isn't pushed down.
You've got "left" stated several times in the CSS, but there's no "top", and you could add that to style.css when the parent li gets :hovered. Top should be something like the height of the menu, or close to it.
You might want to go looking for a theme that's not so old. The code in there is dirty. <center> tags, arg!
This one is still fairly simple as far as theme elements, meaning you wouldn't have to remove a whole lot to get the setup you've got now. Plus it's got those added accessibility features which is nice. Even Yoopers need access. : )
If it doesn't come with a good dropdown menu, the Sons of Suckerfish dropdowns at HTMLdog are nice and basic. If you put it inside the #container box then it would naturally be pretty close to centered. If it was still a bit too far to the left, you could give the menu a small left margin to bump it more center.