#div{
    width: 300px;
    height: 400px;
    background-color: #1D1D1D;
    border: 1px solid #000;
    color: #fff;
}

#screen{
    display: block;
    width: 300px;
    height: 80px;
    margin-top: 16px;
    background-color: #DFE1C9;
}

#btn{

    list-style: none;
    width: 280px;
    height: 280px;
    padding: 0;
    margin-top: 15px;
    position: relative;
    color: #000;
    
}

#btn li{
    display: block;
    width:60px;
    height:  40px;
    float: left;
    margin: 6px 6px 10px 4px;
    line-height: 40px;
    text-align: center;
    font-size: 18px;
    font-weight: 700;
}
#btn li.zero{
    width: 128px;
    height: 40px;
    background-color: #000;
    color: #fff;
}
#btn li:last-child{
    width: 60px;
    height: 95px;
    position: absolute;
    bottom: 0;
    right: 0;
    line-height: 95px;
    background-color: #F6801D;
    color: #fff;
}
.brown{
    background-color: #94877F;
    color: #fff;
}
.black{
    background-color: #000;
    color: #fff;
}