.calculator {
	text-align:center;
	margin:0 auto;
	padding-top:70px;
	width:800px;
}

.calculator-screen {
	width:100%;
	height:100px;
	background-color:#252525;
	color:#fff;
	text-align:right;
	font-size:36px;
	border:none;
	padding: 0 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:#fff;
	width:25%;
	outline:none;
}

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

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

button:hover {
	background-color:darkgray;
}

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