Changing a dialogue box colour

That’s not really a dialog box, really an overlay, if you’re talking about the gray-backgrounded box with the title “Cash” on that page.

You can try making the background of the overlay more opaque by increasing the “0.5” in the code below, which you should add to a child theme’s style.css:

.gallery-single-slider .slide-inner-info {
  background: rgba(0, 0, 0, 0.5) none repeat scroll 0% 0%;
}

The “0.5” really means an opacity of 50%, which is a bit low for this purpose. Eighty percent, or “0.8” would be a good start.