Turns background grey when clicking gallery

It seems that anytime the shortcode has to load, it turns the entire website background grey, messing with the contrast of over images.

I put a preloader in to try and hide the first part, but whenever you click Gallery, it turns the background grey again with the spinning icon, messing with every image on the site. Has anyone seen this before? Just trying to make it look as professional as possible

The page I need help with: [log in to see the link]