Hi Michael, in theory it could be done with some CSS rules that override the script’s settings but the problem is that the script takes the margins into account when calculating the position of the light box and its elements.
When the borders are different then expected, there will most likely occur weird effects due to miscalculations.
That said, you could try adding (and adapting) these style rules to your theme’s style.css or via the Jetpack module Custom CSS:
#fancybox-content {
border-left-width: 10px !important;
border-right-width: 10px !important;
}
@media only screen and (max-width: 640px) {
#fancybox-content {
border-width: 10px !important;
}
}
If you can share a link, I can take a closer look at the result and the unexpected effects that may occur…