.gallery {
    column-count: 3;
    column-gap: 1em;
}

.gallery > div {
    margin-bottom: 1em;
    display: block;
    width: 100%;
    height: auto;
    width: 100%;
    height: auto;
    position: relative;
}

.overlay {
    position: absolute;
    display: flex;
    align-items: center;
    justify-content: center;
    top: 0;
    left: 0;
    height: 100%;
    width: 100%;
    opacity: 0;
    transition: .5s ease;
    background-color: rgba(0, 0, 0, 0.8);
    border-radius: 10px;
}
  
.gallery > div:hover .overlay {
    opacity: 1;
}