qcha0s

The problem is actually your themes h3 setting.

You can adjust this by adding a font-size: attribute to the #tooltip h3, #tooltip div style (around line 19 in lightbox-gallery.css)

was:
#tooltip h3, #tooltip div { margin: 0; }

is now:
#tooltip h3, #tooltip div { margin: 0; font-size: 10px; }

set the 10px to whatever size floats your boat 😉

Cheers!