I had a similar problem with the header on twentyeleven. The solution for that was to set the z-index on the branding item to z-index: 1 and set the z-index for the gallery to something like z-index: 1000.
You’ll need to edit the theme/plugin CSS directly (not recommended) or create a child theme and insert the CSS changes into the style.css file in the child theme.
Given that something in the sidebar is on top of the lightbox, something probably has it’s z-index set to something high in the theme CSS so you’ll probably need to poke around to see what might have a high z-index and then change it to a lower level.
Thread Starter
finc
(@finc)
Thanks,
I added a 0 on all z-index lines in
wp-content/plugins/lightbox-gallery/lightbox-gallery.css
and it worked just fine