body {
    font-family: 'PingFang SC', 'Helvetica Neue', Arial, sans-serif;
    background: #f9f9f9;
    padding-bottom: 20px
}

.category-card {
    border-radius: 16px;
    transition: all .3s
}

.category-card:hover {
    transform: translateY(-3px);
    box-shadow: 0 6px 20px rgba(0, 0, 0, .1)
}

.word-card {
    perspective: 1000px;
    height: 180px
}

.card-inner {
    position: relative;
    width: 100%;
    height: 100%;
    transition: transform .8s;
    transform-style: preserve-3d
}

.word-card:hover .card-inner {
    transform: rotateY(180deg)
}

.card-front,
.card-back {
    position: absolute;
    width: 100%;
    height: 100%;
    backface-visibility: hidden;
    border-radius: 16px;
    box-shadow: 0 4px 15px rgba(0, 0, 0, .05)
}

.card-back {
    transform: rotateY(180deg)
}

.flash-cards-container {
    height: 200px
}

.level-badge {
    padding: 2px 8px;
    border-radius: 10px;
    font-size: 10px;
    background-color: rgba(255, 255, 255, .3);
    color: white
}