body {
    margin: 0 auto;
    width: 90%;
}

.hide {
    display: none;
}

.rectangle {
    /*structure*/
    width: 400px;
    height: 300px;
    
    /* forme */
    background-color: royalblue;
    border-radius: 10px;
    
    transition: background-color 0.8s;
}

.important {
    background-color: firebrick;
}

.good {
    background-color: limegreen;
}