I’m not sure of your specific site’s class for the lightbox, but simply bring up the Inspect Element window from the overlay and it should inform you which class(es) you need to change. This can even be done in the Jetpack Custom CSS.
As for prioritizing over the theme’s default set, write in a !important rule to force that rule to always apply. So for example, if the class you need to change was called .overlay, then:
.overlay {
opacity:0 !important;
}
And that should do it. Since a lightbox overlay may sometimes place its background and imagery into the same class, make sure you test it in the inspector first to see if it makes the entire lightbox transparent.