styles.css

* {
	box-sizing: border-box;
}

body {
	background-color: #3399ff;
	font-family: "Verdana";
	font-size: 120%;
	margin: 0;
}

button {
	font-size: 150%;
	border: 1px solid #000000;
	padding: 9px;
	margin-top: 5px;
	border-radius: 15px;
	cursor: pointer;
}

button:disabled {
	cursor: default;
}

hr {
	margin-top: 1px;
	margin-bottom: 1px;
	border-width: 2px;
}

div.container {
	display: flex;
	justify-content: center;
	margin-top: 50px;
}

div.main {
	background-color: #ffffff;
	padding: 1%;
}

span.error_message {
	color: red;
	font-size: 80%;
	line-height: 1;
}

div.numbers {
	width: 100%;
	height: 76px;
	text-align: right;
}

div.result {
	margin-right: 7px;
}

div.top_buttons, div.main_buttons {
 	margin-top: 5px;
}

button.first_line {
	font-size: 90%;
}

button.backspace {
	padding: 9px 2px;
}

button.minus {
	padding: 9px 12px; 
}