Today I needed to find an answer to this for my new website. In case you still need to remove the white borders from those images (the smaller images in columns underneath the slider image area), it took me a while but I finally found where those borders are coded (under theme-frontpage.php).
Try this in the Custom CSS area:
#front-columns .column-image {
height:px;
border:0px solid #000;
}
The above settings changed the border to black and removed the border entirely.
