style.css

.calculator {
	width: 280px;
	height: 400px;
	margin: auto;
	background-color: #f0f0f0;
	box-shadow: 0 0 10px #ccc;
	border-radius: 5px;
}

.display {
	width: 90%;
	height: 70px;
	padding: 10px;
}

.display input {
	width: 100%;
	height: 100%;
	border: none;
	border-radius: 5px;
	background-color: #fff;
	font-size: 2em;
	text-align: right;
	padding-right: 10px;
	box-shadow: 0 0 5px #ccc;
	outline: none;
}

.buttons {
	display: grid;
	grid-template-columns: repeat(4, 1fr);
	grid-gap: 5px;
	padding: 10px;
}

button {
	font-size: 1.5em;
	padding: 10px;
	background-color: #fff;
	border: 1px solid #ccc;
	cursor: pointer;
	border-radius: 5px;
	box-shadow: 0 0 5px #ccc;
}

.operator {
	background-color: #FF9500;
	color: #fff;
	border-color: #FF9500;
}

.operator:hover {
	background-color: #ffad33;
	border-color: #ffad33;
}

#equals {
	background-color: #4cd964;
	color: #fff;
	border-color: #4cd964;
}

#equals:hover {
	background-color: #5fe075;
	border-color: #5fe075;
}