style.css

html, body, h1, h2, section {
   margin: 0;
   padding: 0;
}
body {
    background-color: #CFD0CF;
    min-width: 600px;
    min-height: 600px;
    font-family: Arial, Helvetica, sans-serif;
    color: #434344;
    user-select: none;
}
.page-wrapper {
    width: 100%;
    height: 100%;
    padding-top: 40px;
}
.title, .timer {
    font-weight: bold;
    text-align: center;
}
.title {
    font-size: 42px;
    line-height: 47px;
    margin-bottom: 40px;
}
.main {
    display: flex;
    justify-content: center;
}
.grid-box {
    perspective: 600px;
    display: grid;
    gap: 25px;
    grid-template-columns: repeat(4, 130px);
    grid-template-rows: repeat(3, 130px);
}
.card {
    display: block;
    position: relative;
    width: 100%;
    height: 100%;
    transition-duration: 500ms;
    transform-style: preserve-3d;
}
.card:hover {
    cursor: pointer;
}
.card-rotated {    
    transition: transform 500ms;
    transform: rotateY(180deg);
}
.front, .back {
    width: 100%;
    height: 100%;
    position: absolute;
    top: 0;
    left: 0;
    backface-visibility: hidden;
    box-sizing: border-box;
    border: 5px solid #fff;
    border-radius: 9px;
    box-shadow: 1px 1px 5px rgba(0, 0, 0, 0.5);
}
.front { 
    display: flex;
    justify-content: center;
    align-items: center;
    font-size: 65px;
    background-color: #fff;
    transform: rotateY(180deg);
}
.matched {
    background-color: #5AD66F;
    border-color: #5AD66F;
}
.not-matched {
    background-color: #F44336;
    border-color: #F44336;
}
.non-clickable {
    pointer-events: none;
}
.back {
    background: linear-gradient(to top right, #22AB93, #19668D);
}
.timer {
    font-size: 32px;
    line-height: 36px;
    margin-top: 30px;
}
.modal {
    display: none;
    position: fixed;
    justify-content: center;
    align-items: center;
    width: 100%;
    height: 100%;

    background-color: rgba(0, 0, 0, 0.5);
    z-index: 1;
}
.visible {
    display: flex;
}
.pop-up {
    min-width: 350px;
    height: auto;
    background-color: #fff;
}
.game-result {
    display: block;
    text-align: center;
    font-size: 48px;
    font-weight: bold;
    margin: 30px 0 0;
}
.game-result span {
	display: inline-block;
	position: relative;
	transform-origin: center bottom;
	animation: letter-jump 1s infinite;
}

.game-result span:nth-child(2) {
	animation-delay: 0.1s;
}

.game-result span:nth-child(3) {
	animation-delay: 0.2s;
}

.game-result span:nth-child(4) {
	animation-delay: 0.3s;
}

@keyframes letter-jump {
	0% {
		transform: scaleY(1);
        animation-timing-function: ease-out;
	}
	25%,35% {
        transform: translateY(-4px) scaleY(1.4);
        animation-timing-function: ease-in;
	}
    90% {
        transform: translateY(0) scaleY(0.6);
        animation-timing-function: ease-in;
	}
	100% {
		transform: translateY(0) scaleY(1);
        animation-timing-function: ease-out;
	}
}

button {
    display: block;
    margin: 40px auto 30px;
    height: 40px;
    font-size: 20px;
    font-weight: bold;
    color: #fff;
    background: linear-gradient(to left, #22AB93, #19668D);
    border: 0;
    border-radius: 10px;
    padding: 0 30px;
    box-shadow: 1px 1px 1px #434344;
    outline: none;
    
}
.clicked {
    box-shadow: inset 2px 1px 8px #434344;
}