html, body {
    height: 100%;
    min-height: 100vh;
}

body {
    display: flex;
    min-height: 100vh;
    flex-direction: column;
}

.announcement {
    display: flex;
    -webkit-justify-content: center;
    justify-content: center;
    margin-bottom: 20px;
}
.announcement .card {
    width: 350px;
    margin-left: 10px;
    margin-right: 10px;
    margin-top: -40px;
}

@media only screen and (max-width: 992px) {
    .announcement {
        flex-flow:column;
        -webkit-justify-content: center;
        justify-content: center;
        margin-bottom: 20px;
    }

        .announcement .card {
            width: 97%;
            margin-top: 10px;
        }
}