How do I change a Gallery lightbox to light instead of the dark default?

I searched and cannot find anything on this subject. On this page, I would like for the lightbox to be light colored when someone clicks on an image, so the initial png graphic stands out better. I'm using the Miller template.

Thank you!

@FetchingWebDesign Add to Home > Design > Custom CSS

.yui3-lightbox2 .sqs-lightbox-slideshow .sqs-lightbox-padder {
    background: #fff;
}

Thank you @tuanphan! That worked great. I ended up using RGBA with opacity set to 0.5 in order to see through it a little.