body {
    padding: 0;
    margin: 0;
}

#level {
    border-spacing: 0;
    border-collapse: collapse;
}

.row {
}

.field {
    border: 0;
    padding: 0;
    margin: 0;
    background-repeat: no-repeat;
    cursor: pointer;
}

.selection {
    width: 100%;
    height: 100%;
    opacity: 0;
    background-color: red;
}

.field:hover .selection {
    opacity: 0.5;
}