:root {
  --bg: #fff8ec; --card: #fff; --ink: #2b2d42; --muted: #6b6f85; --brand: #ef476f; --brand2: #118ab2;
  --ok: #06a77d; --bad: #e63946; --line: #e8dfcf; --soft: #fff1d6;
  font-family: "Segoe UI", "Be Vietnam Pro", Roboto, Arial, sans-serif;
}
* { box-sizing: border-box; }
body { margin: 0; background: var(--bg); color: var(--ink); font-size: 18px; }
header.top { background: linear-gradient(100deg, #e23c66, #f0643c); color: #fff; padding: 14px 20px; box-shadow: 0 6px 18px -12px rgba(120,40,30,.6); display: flex; align-items: center; gap: 16px; flex-wrap: wrap; }
header.top h1 { margin: 0; font-size: 24px; letter-spacing: .5px; }
header.top a { color: #fff; font-weight: 700; text-decoration: none; background: rgba(255,255,255,.22); padding: 7px 14px; border-radius: 999px; transition: background .15s; }
header.top a:hover { background: rgba(255,255,255,.34); }
header.top .sp { flex: 1; }
main { max-width: 980px; margin: 0 auto; padding: 18px 16px 60px; }
h2 { margin: 18px 0 10px; }
.muted { color: var(--muted); font-size: 15px; }

/* thẻ chủ đề */
.topics { display: grid; grid-template-columns: repeat(auto-fill, minmax(230px, 1fr)); gap: 12px; }
.topic { background: var(--card); border: 2px solid var(--line); border-radius: 16px; padding: 14px; cursor: pointer; text-align: left; font: inherit; color: inherit; transition: transform .1s, border-color .1s; }
.topic:hover { transform: translateY(-2px); border-color: var(--brand2); }
.topic.sel { border-color: var(--brand); background: #fff0f3; }
.topic b { display: block; font-size: 18px; margin-bottom: 4px; }
.topic span { font-size: 14px; color: var(--muted); }
.topic .stars { color: #ffb703; font-size: 15px; margin-top: 6px; display: block; }

.levels { display: flex; gap: 8px; flex-wrap: wrap; }
.lv { border: 2px solid var(--line); background: #fff; padding: 8px 14px; border-radius: 999px; cursor: pointer; font: inherit; }
.lv.sel { background: var(--brand2); color: #fff; border-color: var(--brand2); }
.btn { background: var(--brand); color: #fff; border: 0; border-radius: 999px; padding: 12px 26px; font: inherit; font-weight: 700; cursor: pointer; font-size: 18px; }
.btn.alt { background: var(--brand2); } .btn.ghost { background: #fff; color: var(--ink); border: 2px solid var(--line); }
.btn:disabled { opacity: .45; cursor: default; }

/* thẻ bài */
.card { background: var(--card); border: 2px solid var(--line); border-radius: 20px; padding: 20px 22px; margin: 14px 0; }
.card .prompt { font-weight: 700; font-size: 20px; margin-bottom: 14px; }
.progress { height: 10px; background: var(--line); border-radius: 99px; overflow: hidden; margin: 6px 0 14px; }
.progress i { display: block; height: 100%; background: var(--ok); transition: width .3s; }
.feedback { min-height: 30px; font-weight: 700; margin: 10px 0; }
.feedback.ok { color: var(--ok); } .feedback.bad { color: var(--bad); }

/* ô trả lời */
.blank { font: inherit; font-weight: 700; text-align: center; width: 2.6em; height: 2em; border: 2px solid var(--brand2); border-radius: 10px; background: #f2fbff; color: var(--ink); padding: 0; vertical-align: middle; }
select.blank { width: 3em; appearance: none; -webkit-appearance: none; }
.blank:focus { outline: 3px solid #ffd166; outline-offset: 1px; }
.blank.ok { border-color: var(--ok); background: #e6fff6; } .blank.bad { border-color: var(--bad); background: #ffeceb; }
.choices { display: flex; flex-wrap: wrap; gap: 10px; margin-top: 12px; }
.choice { font: inherit; border: 2px solid var(--line); background: #fff; border-radius: 12px; padding: 8px 16px; cursor: pointer; }
.choice.pick { border-color: var(--brand2); background: #e3f6ff; }
.choice.ok { border-color: var(--ok); background: #e6fff6; } .choice.bad { border-color: var(--bad); background: #ffeceb; }

/* nội dung bài dùng chung */
.q { margin: 6px 0 10px; font-size: 20px; }
.ansline { margin-top: 12px; font-size: 20px; }
.num { display: inline-block; min-width: 1.4em; text-align: center; font-weight: 700; }
.eqgrid { display: grid; grid-template-columns: repeat(auto-fit, minmax(240px, 1fr)); gap: 10px 22px; }
.eq { font-size: 24px; line-height: 2.2; white-space: nowrap; }
.eq.big { font-size: 28px; }
.cmpgrid { display: grid; grid-template-columns: repeat(auto-fit, minmax(210px, 1fr)); gap: 10px 22px; }
.cmp, .opq { font-size: 24px; line-height: 2.2; white-space: nowrap; }
.seqgrid .seq { font-size: 24px; line-height: 2.4; }
.seq .sep { margin-right: 2px; }
.bondrow { display: flex; flex-wrap: wrap; gap: 22px; }
.bond { display: flex; flex-direction: column; align-items: center; gap: 6px; padding: 8px 10px; border: 2px dashed #9bb; border-radius: 14px; }
.bond .bt { }
.bond .bb { display: flex; gap: 14px; }
.bond .num { background: #fff3c4; border-radius: 10px; padding: 2px 8px; font-size: 24px; }

/* tổng kết */
.sum { text-align: center; } .sum .big { font-size: 56px; margin: 6px 0; }
.hide { display: none !important; }
footer { text-align: center; color: var(--muted); font-size: 13px; padding: 20px; }

/* mũi tên phép tính, Sudoku mini */
.ac { display: flex; flex-wrap: wrap; align-items: center; gap: 6px; font-size: 22px; }
.ac-op { color: #e76f51; font-weight: 700; font-size: 18px; padding: 0 2px; }
.ac-box { border: 2px solid #999; border-radius: 8px; padding: 2px 8px; background: #fff3c4; }
table.lat { border-collapse: collapse; margin: 4px 0; }
table.lat td { width: 46px; height: 46px; border: 2px solid #555; text-align: center; font-size: 22px; }
table.lat .blank { width: 36px; height: 36px; }

.topics .subj { grid-column: 1 / -1; margin: 10px 0 0; font-size: 20px; }

/* ô chưa điền, dãy số không ngắt giữa chừng, ô chọn dấu có mũi tên, ô danh sách, sơ đồ 2 tầng */
.blank.need { border-color: #ffb703; background: #fff7d6; animation: pulse .8s 2; }
.seq .si { white-space: nowrap; display: inline-block; margin: 2px 0; }
select.blank { padding-right: 14px; background: #f2fbff url("data:image/svg+xml,%3Csvg xmlns=%27http://www.w3.org/2000/svg%27 width=%2710%27 height=%276%27%3E%3Cpath d=%27M0 0l5 6 5-6z%27 fill=%27%23118ab2%27/%3E%3C/svg%3E") no-repeat right 4px center; width: 3.2em; cursor: pointer; }
.sheet-mode select.blank { background-image: none; color: transparent; }
.blank.list { width: 11em; text-align: left; padding: 0 8px; font-weight: 600; }
.sheet-mode .blank.list { width: 70%; }
.bond.two { padding: 10px; } .bond.sub { border-style: dotted; padding: 6px; }
.bond .bb { align-items: flex-end; }

/* chọn nhiều đáp án */
.multi { display: flex; flex-wrap: wrap; gap: 10px; margin-top: 12px; }
.mopt { font: inherit; border: 2px solid var(--line); background: #fff; border-radius: 12px; padding: 8px 14px; cursor: pointer; }
.mopt.on { border-color: var(--brand2); background: #e3f6ff; box-shadow: 0 0 0 3px #b9e6fb inset; }
.mopt.ok { border-color: var(--ok); background: #e6fff6; } .mopt.bad { border-color: var(--bad); background: #ffeceb; } .mopt.miss { border-style: dashed; border-color: var(--ok); }
/* nối hai cột */
.match { position: relative; display: flex; justify-content: space-between; gap: 40px; margin: 8px 0; }
.mcol { display: flex; flex-direction: column; gap: 10px; flex: 1; min-width: 0; z-index: 1; }
.mrow { display: flex; align-items: center; gap: 8px; justify-content: space-between; border: 2px solid var(--line); border-radius: 12px; padding: 6px 10px; background: #fff; cursor: pointer; min-height: 48px; }
.mright .mrow { justify-content: flex-start; }
.mrow.sel { border-color: var(--brand2); box-shadow: 0 0 0 3px #b9e6fb; }
.mlet { color: var(--brand2); }
svg.mlines { position: absolute; left: 0; top: 0; pointer-events: none; z-index: 0; }
svg.mlines line { stroke: #118ab2; stroke-width: 3; stroke-linecap: round; }
@media (max-width: 520px) { .match { gap: 18px; } .mrow { padding: 4px 6px; } }