To set the transparency of the ligthbox go to Appearance > Customize > Additional CSS and add the following CSS code:

For background transparency between 0 (100% transparent) and 0 (no transparency/opaque)

.pswp__bg {
opacity: 0.5 !important;
}

To change the background color to white for example (you may also choose any other color from the HEX table https://www.color-hex.com/ replace the #fff with the new HEX):

.pswp__bg {
background: #fff;
}

Or use both:

.pswp__bg {
opacity: 0.5 !important;
background: #fff;
}