Change following CSS styles customization.

Baguette Box Lightbox

Lightbox Background Color:

#baguetteBox-overlay {
    background-color: rgba(0, 0, 0, 0.8) !important;
}

Caption Color, Font, and Background Color:

#baguetteBox-overlay .full-image figcaption {
    color: #ccc !important;
    background-color: rgba(0,0,0,.6) !important;
    font-family: sans-serif !important;
}

Button Color:

.baguetteBox-button svg g {
    stroke: rgb(160,160,160) !important;
}

Button Background Color:

.baguetteBox-button {
    background-color: rgba(50,50,50,.8) !important;
}

Photoswipe Lightbox

Lightbox Background Color:

.pswp {
    background-color: rgba(0, 0, 0, 0) !important;
}

Caption Color and Font:

.pswp__dynamic-caption {
    color: #fff !important;
}

Button Color:

.pswp__icn {
    fill: #fff !important;
}