Post headers won't fade behind lightbox

I recently installed jQuery Lightbox for Native Galleries to create a portfolio page for my blog. However the theme (Blissful Blog) evidently employs some piece of code that is interfering with the lightbox’s attempt to fade the page when it pops up (see here when you click on a thumbnail). Any ideas on what I need to look for to remove? I thought maybe it had to do with a font-replacement feature of the theme but I am not terribly CSS savvy and I don’t want to go stomping around in the code until I have a better idea of what I’m looking for.

Many thanks to anyone who can be of assistance! This is the best option I have tried so far for my portfolio page and I really don’t want to give up on it for such a small issue.