.card-holder {
    display: flex;
    float: left;
    flex-direction: column;
    background-color: red;
    width: 300px;
    height: calc(100vh - 3.5rem);
}

.card {
    background-color: blue;
    margin: 5px;
    height: 45px;
    color: white;
    align-items: center;
    display: flex;
    justify-content: center;
}

.main-area {
    float: left;
    background-color: aqua;
    height: calc(100vh - 3.5rem);
    width: calc(100vw - 300px);
}