app.css

body {
	margin: 15px;
	background: white;
	font-size: 16px;
	font-family: Tahoma, Geneva, sans-serif;
}

h1 {
	margin: 15px 0;
	text-align: center;
	font-size: 22px;
	font-weight: normal;
}

#board {
	display: flex;
	flex-wrap: wrap;
	justify-content: center;
	align-items: flex-start;
	text-align: center;
	margin: 0 auto;
	max-width: 400px;
}

#board > .item {
	display: flex;
	justify-content: center;
	align-items: center;
	width: 60px;
	height: 60px;
	margin: 5px;
	color: white;
	border: solid 1px #eee;
	cursor: pointer;
	background-color: green;
	border: solid 1px green;
	transition: all 0.5s ease;
}

#board > .item > b {
	transition: opacity 0.5s;
	opacity: 0;
	font-size: 24px;
    -webkit-user-select: none;
    -moz-user-select: none;
    -ms-user-select: none;
    user-select: none;
}
#board > .item > b.active {
	opacity: 1;
}

#board > .item:hover,
#board > .item:focus {
	background-color: yellow;
}

#board > .item.opened,
#board > .item.opened:hover,
#board > .item.opened:focus {
	background-color: blue;
}

#board > .item.done,
#board > .item.done:hover,
#board > .item.done:focus {
	background-color: red;
	cursor: default;
}

#result {
	text-align: center;
	font-size: 14px;
	line-height: 20px;
}