White border problem

There’s not a border on the slideshow, the container that the slideshow is in is styled by the theme with a white background, there is also some padding to add some space, that is the white you are seeing.

Because this div contains more than just the slideshow and is probably used on other pages as well, styling it might be tough. I would try moving the slideshow out of that container, placing it between the container and the nav above it. But the slideshow is narrower than the site, you might want to change the width to 960px to get it to fit properly, but you’d need to resize the slide images and re-upload them.