:root {
  --bg: #fff8ef; --ink: #2d2a4a; --muted: #6b6889; --card: #fff; --line: #f0e6d8;
  --pri: #ff7a45; --pri2: #ffb347; --blue: #4f7cff; --ok: #2e9e5b; --bad: #e5484d;
  --r: 18px; --sh: 0 6px 20px rgba(80, 60, 30, .08);
}
* { box-sizing: border-box; }
body { margin: 0; font-family: 'Nunito', system-ui, sans-serif; background: var(--bg); color: var(--ink); line-height: 1.55; font-size: 17px; }
a { color: inherit; text-decoration: none; }
h1, h2, h3 { line-height: 1.25; margin: .4em 0; }
h2 { font-size: 1.5rem; margin-top: 1.6em; }
.wrap { max-width: 1080px; margin: 0 auto; padding: 0 18px; }
.center { text-align: center; } .muted { color: var(--muted); } .hidden { display: none !important; }

/* Thanh điều hướng */
.nav { position: sticky; top: 0; z-index: 20; background: rgba(255, 248, 239, .92); backdrop-filter: blur(8px); border-bottom: 1px solid var(--line); }
.navin { display: flex; align-items: center; justify-content: space-between; height: 60px; gap: 10px; }
.logo { font-weight: 900; font-size: 1.25rem; white-space: nowrap; }
.nav nav { display: flex; gap: 4px; overflow-x: auto; }
.nav nav a { padding: 6px 12px; border-radius: 999px; font-weight: 700; color: var(--muted); white-space: nowrap; }
.nav nav a:hover, .nav nav a.on { background: #ffe7d6; color: var(--pri); }

/* Nút */
.btn { display: inline-flex; align-items: center; gap: 6px; border: 2px solid var(--line); background: #fff; color: var(--ink); font: inherit; font-weight: 800; padding: 8px 16px; border-radius: 14px; cursor: pointer; transition: .15s; }
.btn:hover { transform: translateY(-2px); box-shadow: var(--sh); }
.btn.primary { background: linear-gradient(135deg, var(--pri), var(--pri2)); border-color: transparent; color: #fff; }
.btn.big { padding: 12px 22px; font-size: 1.05rem; }

/* Hero */
.hero { padding: 48px 0 24px; }
.heroin { display: grid; grid-template-columns: 1.3fr 1fr; gap: 32px; align-items: center; }
.hero h1 { font-size: clamp(2rem, 4.5vw, 3rem); font-weight: 900; }
.hl { background: linear-gradient(135deg, var(--pri), #e83e8c); -webkit-background-clip: text; background-clip: text; color: transparent; }
.hero p { color: var(--muted); font-size: 1.1rem; }
.cta { display: flex; gap: 12px; flex-wrap: wrap; margin-top: 20px; }
.heroart { background: linear-gradient(135deg, #7c6cff, #4fb3ff); color: #fff; border-radius: 28px; padding: 40px 24px; text-align: center; font-weight: 800; box-shadow: var(--sh); }
.heroart .emos { font-size: 3.6rem; letter-spacing: 6px; margin-bottom: 10px; animation: bob 3s ease-in-out infinite; }
@keyframes bob { 50% { transform: translateY(-8px); } }

/* Tính năng */
.features { display: grid; grid-template-columns: repeat(3, 1fr); gap: 16px; margin-top: 20px; }
.feat { background: var(--card); border-radius: var(--r); padding: 20px; box-shadow: var(--sh); }
.feat .fe { font-size: 2rem; } .feat h3 { font-size: 1.1rem; } .feat p { color: var(--muted); margin: 0; font-size: .95rem; }

/* Lớp */
.grades { display: grid; grid-template-columns: repeat(auto-fill, minmax(200px, 1fr)); gap: 16px; margin-top: 18px; }
.grade { background: #fff; border-radius: var(--r); padding: 18px; box-shadow: var(--sh); border-top: 6px solid var(--c); transition: .2s; display: block; }
.grade:hover { transform: translateY(-4px); }
.grade .ge { font-size: 2.4rem; } .grade h3 { color: var(--c); }
.grade ul { padding-left: 0; list-style: none; font-size: .88rem; color: var(--muted); margin: 6px 0 12px; }
.grade li { padding: 2px 0; }
.bar { height: 8px; background: #f1ebe2; border-radius: 99px; overflow: hidden; margin: 6px 0 2px; }
.bar i { display: block; height: 100%; background: linear-gradient(90deg, var(--ok), #7bd88f); border-radius: 99px; }
small { color: var(--muted); }

/* FAQ */
.faq details { background: #fff; border-radius: 14px; padding: 14px 18px; margin: 10px auto; max-width: 760px; box-shadow: var(--sh); }
.faq summary { font-weight: 800; cursor: pointer; }
.faq p { margin: 8px 0 0; color: var(--muted); }
footer { margin-top: 60px; padding: 24px 0; border-top: 1px solid var(--line); color: var(--muted); text-align: center; font-size: .9rem; }

/* Trang lớp / chủ đề */
.back, .crumbs a { display: inline-block; margin: 18px 0 8px; color: var(--blue); font-weight: 700; }
.banner { background: linear-gradient(135deg, var(--c), color-mix(in srgb, var(--c) 55%, #fff)); color: #fff; border-radius: 24px; padding: 24px 28px; box-shadow: var(--sh); }
.banner small { color: rgba(255, 255, 255, .85); font-weight: 800; letter-spacing: 1px; }
.banner h1 { font-size: 2rem; margin: 4px 0; } .banner p { margin: 0; opacity: .9; }
.units { display: grid; gap: 14px; margin-top: 18px; }
.unit { display: grid; grid-template-columns: 64px 1fr auto; gap: 16px; align-items: center; background: #fff; border-radius: var(--r); padding: 16px 18px; box-shadow: var(--sh); border-left: 6px solid var(--c); }
.unit .ue { font-size: 2.4rem; text-align: center; }
.unit h3 { margin: 0; } .unit p { margin: 0; color: var(--muted); font-size: .95rem; }
.unit .ul { display: flex; gap: 8px; flex-wrap: wrap; }

.tabs { display: flex; gap: 8px; margin: 18px 0; flex-wrap: wrap; }
.tabs a { padding: 10px 18px; border-radius: 14px; background: #fff; font-weight: 800; box-shadow: var(--sh); }
.tabs a.on { background: var(--blue); color: #fff; }

.card { background: #fff; border-radius: var(--r); padding: 18px 22px; box-shadow: var(--sh); margin: 14px 0; }
.goals { background: #fffbe6; border: 2px dashed #ffd666; }
.goals ul { margin: 0; padding-left: 22px; }
.theory h3 { color: var(--blue); }
.ex { background: #f3f6ff; border-radius: 12px; padding: 10px 14px; font-weight: 800; font-size: 1.1rem; margin: 8px 0; }
.tip { background: #fff4e5; border-left: 4px solid var(--pri2); border-radius: 8px; padding: 8px 12px; margin: 8px 0; }
.calc { background: #f8f8fc; border-radius: 12px; padding: 10px 18px; font-size: 1.15rem; font-weight: 700; display: inline-block; font-family: ui-monospace, Consolas, monospace; }
.numrow { display: flex; flex-wrap: wrap; gap: 8px; }
.numrow span { background: #f3f6ff; border-radius: 12px; padding: 6px 10px; text-align: center; min-width: 56px; }
.numrow b { display: block; font-size: 1.4rem; color: var(--blue); }
.table { display: grid; grid-template-columns: repeat(5, 1fr); gap: 6px; margin: 8px 0 14px; }
.table span { background: #f8f8fc; border-radius: 8px; padding: 4px 8px; text-align: center; font-size: .95rem; }
.table b { color: var(--pri); }
.tbl { border-collapse: collapse; width: 100%; font-size: .85rem; text-align: center; }
.tbl th, .tbl td { border: 1px solid var(--line); padding: 6px 4px; } .tbl th { background: #f3f6ff; }
.example .eq { font-weight: 800; }
.example summary { cursor: pointer; color: var(--blue); font-weight: 700; margin-top: 6px; }
.next { text-align: center; margin: 24px 0; }
.steps { display: flex; gap: 8px; flex-wrap: wrap; margin-top: 10px; }
.steps a { background: #fff; border: 2px solid var(--line); border-radius: 999px; padding: 4px 12px; font-weight: 700; font-size: .9rem; color: var(--muted); cursor: pointer; }
.steps a:hover { border-color: var(--blue); color: var(--blue); }
h2[id^="s"] { scroll-margin-top: 70px; }
.tag { display: inline-block; background: var(--pri); color: #fff; border-radius: 8px; padding: 0 8px; font-size: .85rem; margin-right: 4px; }
.guide { background: #f3f6ff; border-radius: 12px; padding: 10px 14px; margin-top: 8px; }
.worked details { margin-top: 8px; background: #fafaff; border-radius: 12px; padding: 8px 12px; }
.worked summary { cursor: pointer; font-weight: 800; color: var(--blue); }
.sol { margin: 8px 0; padding-left: 22px; } .sol li { margin: 4px 0; }
.dap { font-weight: 800; color: var(--ok); }

/* Phân số */
.fr { display: inline-flex; flex-direction: column; vertical-align: middle; text-align: center; line-height: 1.1; margin: 0 3px; font-weight: 800; }
.fr span:first-child { border-bottom: 2px solid currentColor; padding: 0 3px; }
.sq { white-space: nowrap; } .sq sup { font-size: .65em; margin-right: -3px; }
.sq > span { border-top: 1.5px solid currentColor; padding: 0 2px; margin-left: 1px; }
.sys { display: inline-flex; flex-direction: column; vertical-align: middle; border-left: 2px solid currentColor; border-radius: 8px 0 0 8px; padding: 2px 0 2px 8px; margin: 2px 4px; line-height: 1.5; }
.fig { display: block; margin: 10px auto; max-width: 100%; height: auto; }
.fig text { font: 600 14px Nunito, sans-serif; fill: currentColor; }
h2.chap { color: var(--ink); border-left: 6px solid #e53935; padding-left: 10px; font-size: 1.25rem; margin-top: 1.4em; }
.emo { font-size: 1.8rem; letter-spacing: 4px; margin-top: 6px; word-break: break-all; }

/* Bài tập */
.best { background: #fffbe6; border-radius: 12px; padding: 8px 14px; font-weight: 700; margin-bottom: 10px; }
.quiz { padding-left: 0; list-style: none; counter-reset: q; margin: 0; }
.qi { counter-increment: q; border: 2px solid var(--line); border-radius: 14px; padding: 12px 14px; margin: 10px 0; transition: .2s; }
.qi.ok { border-color: var(--ok); background: #effaf3; }
.qi.bad { border-color: var(--bad); background: #fff1f1; }
.qhead { display: flex; gap: 10px; align-items: flex-start; }
.qtext { flex: 1; font-weight: 700; }
.qtext::before { content: "Câu " counter(q) ". "; color: var(--pri); font-weight: 900; }
.choices { display: flex; flex-wrap: wrap; gap: 8px; margin-top: 10px; }
.ch { min-width: 64px; border: 2px solid var(--line); background: #fff; border-radius: 12px; padding: 8px 14px; font: inherit; font-weight: 800; cursor: pointer; font-size: 1.1rem; }
.ch:hover { border-color: var(--blue); }
.ch.sel { background: var(--blue); color: #fff; border-color: var(--blue); }
.ans { display: flex; gap: 8px; align-items: center; margin-top: 10px; flex-wrap: wrap; }
.inp { font: inherit; font-weight: 800; font-size: 1.1rem; padding: 8px 12px; border: 2px solid var(--line); border-radius: 12px; width: 160px; }
.inp:focus { outline: none; border-color: var(--blue); }
.unit { font-weight: 700; color: var(--muted); }
.fb { margin-top: 6px; font-weight: 700; min-height: 1px; }
.qi.ok .fb { color: var(--ok); } .qi.bad .fb { color: var(--bad); }
.qbar { display: flex; gap: 10px; flex-wrap: wrap; margin: 14px 0; }
.res-card { text-align: center; background: linear-gradient(135deg, #e8f0ff, #fff3e6); border-radius: 16px; padding: 16px; font-size: 1.15rem; font-weight: 700; }
.res-card b { font-size: 1.8rem; color: var(--pri); }

/* Đề luyện & trò chơi */
.tests, .games { display: grid; grid-template-columns: repeat(auto-fill, minmax(150px, 1fr)); gap: 12px; }
.games { grid-template-columns: repeat(auto-fill, minmax(220px, 1fr)); }
.test, .game { background: #fff; border-radius: 16px; padding: 14px; box-shadow: var(--sh); text-align: center; display: flex; flex-direction: column; gap: 2px; border: 3px solid transparent; transition: .15s; }
.test:hover, .game:hover { transform: translateY(-3px); }
.test.on, .game.on { border-color: var(--blue); }
.test.hard b { color: var(--bad); }
.test span, .game span { font-size: 1.8rem; }
.game em { font-style: normal; font-weight: 800; color: var(--pri); font-size: .9rem; }

.gamebox { margin-top: 18px; min-height: 300px; }
.gtop { display: flex; justify-content: space-between; gap: 10px; font-weight: 800; font-size: 1.1rem; background: #f3f6ff; padding: 8px 14px; border-radius: 12px; }
.gq { font-size: 1.4rem; font-weight: 800; text-align: center; margin: 18px 0 10px; }
#ga { display: flex; justify-content: center; } #ga .choices { justify-content: center; }
.gfb { text-align: center; font-weight: 800; min-height: 1.6em; margin-top: 10px; }
.gfb.ok { color: var(--ok); } .gfb.bad { color: var(--bad); }
.gend { text-align: center; padding: 20px; } .big-emo { font-size: 4rem; }

.sky { position: relative; height: 380px; background: linear-gradient(#bfe6ff, #eaf7ff); border-radius: 16px; overflow: hidden; }
.balloon { position: absolute; bottom: -130px; width: 92px; height: 112px; border-radius: 50% 50% 46% 46%; border: none; color: #fff; font: inherit; font-weight: 900; font-size: 1.15rem; cursor: pointer; animation: rise linear forwards; box-shadow: inset -10px -10px 0 rgba(0, 0, 0, .1); }
.balloon::after { content: ""; position: absolute; left: 50%; bottom: -26px; width: 2px; height: 26px; background: #888; }
.balloon span { display: block; padding: 0 4px; }
@keyframes rise { to { bottom: 110%; } }
.balloon.pop { animation-play-state: paused; transform: scale(1.4); opacity: 0; transition: .3s; pointer-events: none; }
.balloon.wrong { filter: grayscale(1); }

.cards { display: grid; grid-template-columns: repeat(4, 1fr); gap: 10px; margin-top: 14px; }
.mcard { position: relative; height: 104px; border: none; background: none; perspective: 600px; cursor: pointer; font: inherit; padding: 0; }
.mcard .front, .mcard .back { position: absolute; inset: 0; border-radius: 14px; display: flex; align-items: center; justify-content: center; backface-visibility: hidden; transition: transform .4s; padding: 6px; font-weight: 700; font-size: .95rem; }
.mcard .front { background: linear-gradient(135deg, #7c6cff, #4fb3ff); color: #fff; font-size: 2rem; font-weight: 900; }
.mcard .back { background: #fff; border: 3px solid #d9d4ff; transform: rotateY(180deg); }
.mcard.a .back { background: #fff8e1; border-color: #ffd666; font-size: 1.3rem; }
.mcard.flip .front { transform: rotateY(180deg); } .mcard.flip .back { transform: rotateY(0); }
.cards.small .mcard { height: 130px; }
.cards.small .mcard .back { font-size: .78rem; line-height: 1.3; overflow: hidden; }
.cards.small .mcard.a .back { font-size: 1.1rem; }
.mcard.done .back { border-color: var(--ok); background: #effaf3; }

.tfans { text-align: center; font-size: 1.4rem; background: #fff8e1; border-radius: 14px; padding: 10px; }
.tfbtn { display: flex; justify-content: center; gap: 14px; margin-top: 16px; }

.confetti { position: fixed; inset: 0; pointer-events: none; z-index: 99; overflow: hidden; }
.confetti span { position: absolute; top: -40px; animation: fall 2.4s ease-in forwards; }
@keyframes fall { to { transform: translateY(110vh) rotate(540deg); } }

@media (max-width: 760px) {
  .heroin, .features { grid-template-columns: 1fr; }
  .unit { grid-template-columns: 48px 1fr; } .unit .ul { grid-column: 1 / -1; }
  .table { grid-template-columns: repeat(2, 1fr); }
  .cards { grid-template-columns: repeat(3, 1fr); }
  .logo span { display: none; }
  .balloon { width: 72px; height: 90px; font-size: .95rem; }
}