main.css

* {
    margin: 0;
    padding: 0;
}

body {
    width: 100%;
    height: 80vh;
    display: flex;
    overflow: hidden;
    user-select: none;
    position: relative;
    align-items: center;
    animation-name: init;
    flex-direction: column;
    animation-duration: 1s;
    justify-content: center;
    background-color: #21252b;
}

.container {
    width: 310px;
    display: flex;
    flex-wrap: wrap;
    margin-bottom: 16px;
    align-items: center;
    justify-content: center;
}

.box {
    color: #fff;
    width: 100px;
    height: 100px;
    display: flex;
    font-size: 30px;
    cursor: pointer;
    user-select: none;
    position: relative;
    align-items: center;
    justify-content: center;
    transition-duration: 0.3s;
    border-left: 2px solid #fff;
    border-bottom: 2px solid #fff;
}

.box:hover {
    background-color: #000000;
}

.box:nth-child(1),
.box:nth-child(2) {
    border-top: 2px solid #fff;
}

.box:nth-child(6),
.box:nth-child(9) {
    border-right: 2px solid #fff;
}

.box:nth-child(3) {
    border-top: 2px solid #fff;
    border-right: 2px solid #fff;
}

.blue::after {
    content: ' ';
    width: 60px;
    height: 60px;
    position: absolute;
    border-radius: 100px;
    background-color: #29B6F6;
}

.yellow::after {
    content: ' ';
    width: 60px;
    height: 60px;
    position: absolute;
    border-radius: 100px;
    background-color: #FFEE58;
}

.btns {
    width: 310px;
    display: flex;
    margin: 0 auto;
    align-items: center;
    justify-content: center;
}

.btns button {
    border: none;
    color: #fff;
    outline: none;
    cursor: pointer;
    padding: 0 26px;
    font-size: 16px;
    user-select: none;
    line-height: 36px;
    border-radius: 4px;
    transition-duration: 0.3s;
    background-color: #455A64;
}

.btns button:hover {
    background-color: #263238;
}

.mask {
    top: 0;
    left: 0;
    z-index: 1;
    width: 100%;
    height: 100vh;
    display: none;
    color: #fff;
    font-size: 60px;
    user-select: none;
    text-align: center;
    position: absolute;
    align-items: center;
    flex-direction: column;
    justify-content: center;
    background-color: rgba(0, 0, 0, 0.9);
}

.is-show {
    display: flex;
    animation: init 2s;
}

.play-btn {
    border: none;
    color: #fff;
    cursor: pointer;
    font-size: 16px;
    padding: 0 26px;
    margin-top: 26px;
    line-height: 36px;
    border-radius: 4px;
    transition-duration: 0.3s;
    background-color: #4CAF50;
}

.play-btn:hover {
    background-color: #2E7D32;
}

.winner {
    width: 120px;
    height: 120px;
    margin-bottom: 30px;
    border-radius: 100px;
}

.bg-blue {
    background-color: #29B6F6;
}

.bg-yellow {
    background-color: #FFEE58;
}

@keyframes init {
    from {transform: translateY(50px); opacity: 0;}
    to {transform: translateY(0); opacity: 1;}
}