a.popup {
    color: cornflowerblue;
}
div.popup {
    display: none;
    height: 100vh;
    width: 100%;
    background: black;
    opacity: 0.9;
    position: fixed;
    top: 0;
    left: 0;
}
div.content{
    background: #fff;
    padding: 30px;
    width: 50%;
}
div.show {
    display: flex;
    justify-content: center;
    align-items: center;
}
div.error-message {
    color: red;
}