.container {
    height: 100%;
    min-height: 100%;
    display: flex;
    flex-direction: column;
}

.box {
    padding: 20px;
    display: flex;
    flex-direction: column;
}

.mainbox {
    width: 100%;
    height: 0;
    padding-top: 100%;
    background-color: #cccccc;
    position: relative;
    box-sizing: border-box;
}

.navbar {
    height:120px; 
    margin-top:30px; 
    background:gray; 
    border-bottom:lightgreen; 
    border-width:10px;
}

.content {
    background:beige; 
    margin:10px; 
    border:mintcream; 
    border-width:2px; 
    flex:1;
}