style.css

.popupviewerContentStyle() {
    box-shadow: 0.2em 0.2em 3em rgba(255, 255, 255, 0.4);
    border-radius: 6px;
}

#popupviewer, #popupviewer:before {
    position: fixed;
    top: 0; left: 0; right: 0; bottom: 0;
    z-index: 11000;
}

#popupviewer:before {
    content: '';
    background-color: rgba(0,0,0,0.8);
    pointer-events: none;
}

#popupviewer > .controls > a {

    background: url(images/modifier-images.png);
    position: absolute;
    cursor: pointer;
    width: 24px;
    display: none;
    height: 24px;

    top: 0px;
    right: 0px;

    margin-top: -13px;
    margin-right: -13px;
    z-index: 11010;

    &.next {
        background-position: -24px 0;
        margin-right: 26px;
    }

    &.close {
        background-position: -48px 0;
    }

    &.previous {
        margin-right: 50px;
    }
    
    &.inactive {
        opacity: 0.5;
        cursor: default;
    }
    
    &.visible {
        display: block;
    }
}

#popupviewer > .controls > .content.isImage > a {
    display: block;
}

#popupviewer > .controls > .content.isImage img {
    .popupviewerContentStyle();
}

#popupviewer > .controls > .content {
    background-color: #fff;
    border: 2px solid #999;
    z-index: 11002;
    padding: 10px;
    overflow: auto;
    box-sizing: border-box;
    .popupviewerContentStyle();
    
    &.isImage, &:empty {
        padding: 0px;
        border: none;
        background-color: transparent;
        overflow: visible;
    }
    
    &:empty {
        background: url(images/loading.gif);
        width: 208px;
        height: 13px;
        margin: auto;
        position: relative;
    }
    
    &:empty ~ a.next,
    &:empty ~ a.previous {
        display: none;
    }
}

#popupviewer > .controls > .additionalContent {

    .popupviewerContentStyle();
    background-color: #fff;
    padding: 0px 30px;
    position:relative;
    border: 1px solid #333;
    
    &:empty {
        display: none;
    }
}

#popupviewer > .controls {
    position:absolute;

    /* by default it is empty thus showing the loader */
    left:50%;
    top:50%;
    margin-left: -104px;

    z-index: 11004;
}