.container {
    width: 960px;
    height: 500px;
    position: relative;
}
.red {
    background: darkred;
    width: 200px;
    height: 200px;
    position: absolute;
    top: 0;
    left: 0;
}

.modal {
    background: snow;
    text-align: center;
    width: 480px;
    height: 200px;
    position: absolute;
    top: calc(50% - 200px);
    left: calc(50% - 240px);
    display: none;
}

.modal.active {
    display: block;
}

.modal-title {
    padding: 1em;
    color: white;
    background: yellowgreen;
}

.modal-active {
    background: rgba(0, 0, 0, 0.5);
}