.bildboxklein {
    width: 450px;
    height: 300px;
    overflow: hidden;
    display: inline-block;
    margin: 12px;
}

.bildboxklein img {
    width: 100%;
    height: 100%;
    object-fit: cover;
    display: block;
}

.lightbox {
    display: none;
    position: fixed;
    z-index: 9999;
    left: 0;
    top: 0;
    width: 100%;
    height: 100%;
    background: rgba(0,0,0,0.9);
    align-items: center;
    justify-content: center;
}

 .lightbox-content {
     position: relative;
     display: inline-block;
 }

.lightbox img {
    max-width: 90vw;
    max-height: 85vh;
    display: block;
}

.lightbox button {
    position: absolute;
    top: -12px;
    right: -12px;
    font-size: 26px;
    background: #fff;
    border: none;
    padding: 4px 10px;
    cursor: pointer;
    border-radius: 4px;
}

@media screen and (max-width: 600px) {

    .bildboxklein {
        width: 95%;
        height: 95%;
    }
}