.calculator {
    text-align:center;
    margin:0 auto;
    padding-top: 200px;
    width: 400px;
}

.calculator-screen{
    width:100%;
    height:100px;
    background-color: black;
    color: white;
    text-align: right;
    font-size: 36px;
    border: none;
    padding: 0px 20px;
    box-sizing: border-box;
}

.calculator-keys{
    width: 100%;
}

.row{
    display: flex;
}

button{
    height:80px;
    background-color: gray;
    border: 1px solid black;
    font-size: 32px;
    color: white;
    width: 25%;
    outline: none;
}

.all-clear, .zero-btn{
    width:50%;
}

.operator, .equal-sign{
    background-color: orange;
}

button:hover{
    background-color: darkgray;
}

.operator:hover, .equal-sign:hover{
    background-color: darkorange;
}