How can I change the black background color into white for the slide box images?

See:

:uncategorised&Itemid=101#gallery1e19f96ec8-2

etc.

With Element inspection I can change it like:

.fancybox-slide {
    -webkit-backface-visibility: hidden;
    display: none;
    height: 100%;
    left: 0;
    outline: none;
    overflow: auto;
    -webkit-overflow-scrolling: touch;
    padding: 44px;
    position: absolute;
    text-align: center;
    top: 0;
    transition-property: transform,opacity;
    white-space: normal;
    width: 100%;
    z-index: 99994;
    background-color: white;
}

But where can I find the