style.css

/* style */
body {
	width: 100%;
	height: 100%;
	margin: 0;
	padding: 0;
	font-size: 15px;
	font-family: 'Open Sans', 'Helvetica Neue', sans-serif;
	background-color: #b6c9d6;
}
.container {
	border: 3px solid #000;
	border-radius: 5px;
	width: 400px;
	height: 550px;
	margin: 30px auto 0 auto;
	display: flex;
	flex-direction: column;
}
/* screen style */
.screen {
	display: flex;
	flex-direction: column;
	justify-content: space-between;
	height: 150px;
	width: 100%;
}
.history-screen {
	text-align: end;
	font-size: 1.5rem;
	margin: 20px 15px 0 0;
}
.display {
	text-align: end;
	font-size: 3rem;
	margin: 0 10px 15px 0;
}

/* button style */
.button-side {
	border-top: 3px solid #000;
	display: flex;
	flex-wrap: wrap;
}
.button-side button {
	width: 100px;
	height: 80px;
}
.dark-button {
	font-size: 1.5rem;
	color: #e9e2d0;
	background-color: #333333;
	border: .2px solid #fff;
	outline: none;
}
.dark-button:active {
	opacity: 0.5;
}
.semi-dark-button {
	font-size: 1.5rem;
	color: #e9e2d0;
	background-color: #706c61;
	border: .2px solid #fff;
	outline: none;
}
.semi-dark-button:active {
	opacity: 0.5;
}
.light-button {
	font-size: 1.5rem;
	color: #000;
	background-color: #00d4c9;
	border: .2px solid #fff;
	outline: none;
}
.light-button:active {
	opacity: 0.5;
}
.key-pressed {
	opacity: 0.5;
}