:root {
    --bg-top: #f2f7ff;
    --bg-bottom: #f8f5ff;
    --card-bg: #ffffff;
    --text-main: #1f2937;
    --text-sub: #4b5563;
    --line: #e5e7eb;
    --accent: #0f766e;
    --accent-soft: #ccfbf1;
    --shadow: 0 18px 40px rgba(15, 23, 42, 0.08);
}

* {
    box-sizing: border-box;
}

html {
    background:
        radial-gradient(
            circle at 0% 0%,
            rgba(20, 184, 166, 0.12),
            transparent 30%
        ),
        linear-gradient(160deg, var(--bg-top) 0%, var(--bg-bottom) 100%);
}

body {
    font-family: "Segoe UI", "Hiragino Kaku Gothic ProN", "Meiryo", sans-serif;
    margin: 40px auto;
    width: min(840px, calc(100% - 32px));
    line-height: 1.75;
    color: var(--text-main);
    background: var(--card-bg);
    border: 1px solid var(--line);
    border-radius: 16px;
    box-shadow: var(--shadow);
    padding: 32px;
}

h1,
h2,
h3 {
    line-height: 1.35;
    letter-spacing: 0.01em;
}

h1 {
    margin: 0 0 20px;
    padding-bottom: 12px;
    border-bottom: 2px solid var(--line);
    font-size: clamp(1.7rem, 1.4rem + 1vw, 2.2rem);
}

h2 {
    margin-top: 30px;
    margin-bottom: 10px;
    font-size: clamp(1.2rem, 1.1rem + 0.5vw, 1.45rem);
}

h3 {
    margin-top: 20px;
    margin-bottom: 10px;
    font-size: 1.15rem;
}

p,
li {
    color: var(--text-sub);
}

ul {
    margin-top: 8px;
    padding-left: 1.2em;
}

section {
    margin-top: 18px;
    border: 1px solid var(--line);
    border-radius: 12px;
    padding: 18px 20px;
    background: linear-gradient(180deg, #ffffff 0%, #fbfdff 100%);
}

nav {
    margin-bottom: 28px;
    display: flex;
    flex-wrap: wrap;
    gap: 10px;
}

a {
    color: var(--accent);
}

nav a {
    text-decoration: none;
    color: var(--accent);
    border: 1px solid #99f6e4;
    background: var(--accent-soft);
    padding: 6px 12px;
    border-radius: 999px;
    font-size: 0.95rem;
    transition:
        background-color 0.2s ease,
        transform 0.2s ease;
}

nav a:hover {
    background: #99f6e4;
    transform: translateY(-1px);
}

@media (max-width: 640px) {
    body {
        margin: 18px auto;
        width: calc(100% - 20px);
        padding: 22px;
        border-radius: 12px;
    }

    h1 {
        margin-bottom: 16px;
    }

    nav {
        margin-bottom: 22px;
    }
}