Ok, now if I clear the cache in my browser I can see the background of the fullscreen galleries being white on your website. Please make sure you clear the cache — as your browser may have cached the previous style.
By default the theme is suing #222 for text (almost black) and #fbfbfb for background (almost white).
These are hex codes. The same ones that you see in Photoshop color picker. So you can come up with a color in Photoshop, and then insert it in CSS.
If you enjoy the theme (or the support), please rate it on ThemeForest. This literally takes a moment and creates a huge difference.