style.css

* {
	box-sizing: inherit;
	margin: 0;
	padding: 0;
}

body {
    text-align:center;
    font-family: sans-serif;
    padding:20px;
}

#currentNumber {
    width: 100px;
    height: 100px;
    background: #ececec;
    border-radius: 50%;
    display: flex;
    justify-content: center;
    align-items: center;
    margin:0 auto 10px;
    font-size:40px;
}

#oldNumbers {
    margin:10px auto 0;
    max-width: 300px;
}

#generateNumber {
	border: 0;
	padding: 15px 30px;
	display: inline-block;
	color: #fff;
	text-transform: uppercase;
	border-radius: 10px;
	font-size: 11px;
	font-weight: bold;
	font-family: Arial;
	background: linear-gradient(270deg, #5f2c82, #ee38bf, #ff5858, #49a09d, #1b5ece, #5f2c82, #ee38bf, #ff5858);
	background-size: 400% 400%;
    animation: rainbowEffect 30s ease infinite;
    cursor:pointer;
}
@keyframes rainbowEffect {
	0%   { background-position: 0% center; }
	100% { background-position: 100% center; }
}

#retry {
    display:none;
    margin: 0 auto;
    background: transparent;
    border: 0;
    text-decoration: underline;
    cursor: pointer;
}

#retry.new-game {
    display: block;
}