body {
    font-family: 'Courier New', Courier, monospace;
}

#header {
    border-radius: 10px;
    text-align: center;
    background-color: dimgray;
    width: 100%;
    padding-top: 25px;
    padding-bottom: 25px;
}

#content {
    min-width: 400px;
    max-width: 900px;
    width: 50%;
    margin: 0;
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);    
}

.ab {
    float:none;
    background-color: lightslategrey;
    border-radius: 10px;
    margin-top: 10px;
    justify-content: center;
    align-items: center;
    display: flex;
    padding: 5px;
}

.a {
    float:left;
    width: 45%;
    box-sizing: border-box;
    background-color:gold;
    padding: 25px;
    border-radius: 10px;
}

.b {
    float:left;
    width: 45%;
    box-sizing: border-box;
    background-color: black;
    color: white;
    padding: 25px;
    border-radius: 10px;
}

.separator {
    float:left;
    width: 10%;
    box-sizing: border-box;
    writing-mode: vertical-lr;
    transform:rotate(210deg);
    font-size: xx-small;
    display: flex;
    justify-content: center;
    align-items: center;
}

@media only screen and (max-width:480px) {
    #content {
        min-width: 95%;
        transform: none;
        position: absolute;
        top: 5px;
        left: 50%;
        transform: translate(-50%);    
        }
    
    .a {
        font-size: small;
    }
    .b {
        font-size: small;
    }
}