:root {
    --navy: #10233f;
    --blue: #2367d1;
    --light: #f5f8fc;
    --text: #263548;
    --muted: #637083;
    --white: #ffffff;
    --border: #dfe6ef;
}
* { box-sizing: border-box; }
html { scroll-behavior: smooth; }
body {
    margin: 0;
    font-family: Arial, Helvetica, sans-serif;
    color: var(--text);
    background: var(--light);
    line-height: 1.7;
}
a { color: var(--blue); text-decoration: none; }
a:hover { text-decoration: underline; }
.container {
    width: min(960px, calc(100% - 36px));
    margin: 0 auto;
}
header {
    background: var(--navy);
    color: var(--white);
    padding: 18px 0;
}
header .container {
    display: flex;
    align-items: center;
    justify-content: space-between;
    gap: 20px;
}
header a { color: var(--white); font-weight: 700; }
main { padding: 52px 0; }
article {
    background: var(--white);
    border: 1px solid var(--border);
    border-radius: 18px;
    padding: 34px;
    box-shadow: 0 12px 30px rgba(16, 35, 63, 0.08);
}
h1, h2, h3 { color: var(--navy); line-height: 1.2; }
h1 { font-size: clamp(2rem, 5vw, 3.4rem); margin-top: 0; }
.lead { color: var(--muted); font-size: 1.08rem; }
.notice {
    margin: 24px 0;
    padding: 18px;
    border-left: 5px solid var(--blue);
    background: #eef5ff;
}
.lesson-nav {
    display: flex;
    flex-wrap: wrap;
    justify-content: space-between;
    gap: 12px;
    margin-top: 34px;
}
.button {
    display: inline-block;
    padding: 11px 17px;
    border-radius: 10px;
    background: var(--blue);
    color: var(--white);
    font-weight: 700;
}
.button.secondary {
    background: var(--navy);
}
code, pre {
    font-family: Consolas, "Courier New", monospace;
}
pre {
    overflow-x: auto;
    padding: 18px;
    border-radius: 12px;
    background: #0f1e33;
    color: #edf5ff;
}
footer {
    padding: 30px 0;
    color: var(--muted);
    text-align: center;
}
@media (max-width: 640px) {
    header .container { flex-direction: column; text-align: center; }
    article { padding: 24px; }
}