That was my fault! I can make it happen in Firefox (see attached) or IE10.
I had tried to change the z-index statements on the first line of your black.css and white.css files to 99000 and 99100, as per:
.mb_overlay{z-index:99000;} #mb_gallery_{z-index:99100;} .mb_overlay{
but it didn't seem to have any effect. so I left them changed.
In response to your comment above, I reviewed what I'd done and found it was indeed working. I then changed the z-index values back to their original, closed the browser and reloaded and my problem was back (see attached). I then changed the values as above, closed and reloaded the browser and everything was working as expected.
I don't know if the values I chose above are realistic or not, but I hope this helps.
Regards
David
