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

body {
    font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
    background: #f5f7fa;
    color: #2c3e50;
    line-height: 1.65;
    max-width: 900px;
    margin: 40px auto;
    padding: 0 20px 40px;
}

header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    background: #34495e;
    color: #ecf0f1;
    padding: 20px 30px;
    border-radius: 12px;
    box-shadow: 0 4px 12px rgba(0,0,0,0.1);
}

header .header-left h1 {
    font-weight: 700;
    font-size: 1.8rem;
    letter-spacing: 0.03em;
    margin-bottom: 6px;
}

header .header-left .subtitle {
    font-weight: 400;
    font-size: 1rem;
    color: #bdc3c7;
}

header .github-button {
    background: #ecf0f1;
    color: #34495e;
    padding: 10px 18px;
    border-radius: 6px;
    text-decoration: none;
    font-weight: 600;
    font-size: 1rem;
    box-shadow: 0 2px 8px rgba(0,0,0,0.12);
    transition: background-color 0.3s ease, color 0.3s ease;
}

header .github-button:hover,
header .github-button:focus {
    background: #2980b9;
    color: #fff;
    outline: none;
}

main.container {
    margin-top: 30px;
}

.card {
    background: #fff;
    padding: 30px 30px 35px;
    border-radius: 16px;
    box-shadow: 0 10px 20px rgba(0,0,0,0.08);
    margin-bottom: 40px;
    transition: box-shadow 0.3s ease;
}

.card:hover {
    box-shadow: 0 14px 28px rgba(0,0,0,0.12);
}

h3 {
    font-size: 1.6rem;
    font-weight: 700;
    color: #34495e;
    margin-bottom: 24px;
    border-bottom: 2px solid #2980b9;
    padding-bottom: 10px;
}

p {
    font-size: 1.1rem;
    color: #4d5d6e;
    margin-bottom: 18px;
}

p b.point {
    color: #2980b9;
    font-weight: 700;
}

hr {
    border: none;
    border-bottom: 1px solid #dfe6e9;
    margin: 32px 0;
}

.math p {
    font-size: 1.15rem;
    line-height: 1.5;
    color: #2c3e50;
}

.math .katex-display {
    margin: 1em 0;
    padding: 15px 20px;
    background-color: #f0f4f8;
    border-left: 6px solid #2980b9;
    border-radius: 6px;
    box-shadow: 0 1px 4px rgba(0,0,0,0.05);
}

.math .katex {
    font-size: 1.15rem;
    color: #34495e;
}

@media (max-width: 600px) {
    header {
        flex-direction: column;
        text-align: center;
        padding: 25px 20px;
    }
    header .header-right {
        margin-top: 15px;
    }
    .card {
        padding: 20px 20px 25px;
    }
    h3 {
        font-size: 1.4rem;
    }
    p {
        font-size: 1rem;
    }
}