/* Lớp Học Nhỏ: bảng xanh, phấn vàng, giấy ấm. Một cột đọc rộng tối đa 760px. */
:root{
  --bg:#f7f4ec; --paper:#ffffff; --ink:#1c2a24; --muted:#5f6f68; --line:#dcd7c9;
  --board:#1f5f4a; --board-deep:#174a3a; --chalk:#f6c744; --rose:#e0705b; --sky:#3b8ea5;
  --mg:#e0705b; --l1:#3b8ea5; --l5:#1f5f4a;
  --shadow:0 4px 0 #dcd7c9;
  --display:"Lexend","Be Vietnam Pro",system-ui,sans-serif;
  --body:"Be Vietnam Pro","Segoe UI",system-ui,sans-serif;
}
@media (prefers-color-scheme: dark){:root:not([data-theme="light"]){
  --bg:#141b18; --paper:#1e2823; --ink:#eef2ef; --muted:#a4b2aa; --line:#33403a;
  --board:#2b7a61; --board-deep:#1f5f4a; --chalk:#f7cf5e; --shadow:0 4px 0 #0d1310; color-scheme:dark}}
:root[data-theme="dark"]{
  --bg:#141b18; --paper:#1e2823; --ink:#eef2ef; --muted:#a4b2aa; --line:#33403a;
  --board:#2b7a61; --board-deep:#1f5f4a; --chalk:#f7cf5e; --shadow:0 4px 0 #0d1310; color-scheme:dark}
*{box-sizing:border-box}
html{padding-top:env(safe-area-inset-top,0px);padding-bottom:env(safe-area-inset-bottom,0px)}
body{margin:0;background:var(--bg);color:var(--ink);font-family:var(--body);font-size:17px;line-height:1.55;
  padding-left:max(16px,env(safe-area-inset-left,0px));padding-right:max(16px,env(safe-area-inset-right,0px));padding-block:0}
a{color:var(--board);text-decoration-thickness:2px;text-underline-offset:3px}
img{max-width:100%}
.wrap{max-width:760px;margin:0 auto}
header.top{display:flex;align-items:center;justify-content:space-between;gap:12px;padding-block:14px;flex-wrap:wrap}
.logo{display:flex;align-items:center;gap:10px;text-decoration:none;color:var(--ink);font-family:var(--display);font-weight:800;font-size:1.25rem}
.logo i{width:34px;height:34px;border-radius:9px;background:var(--board);display:inline-grid;place-items:center;color:var(--chalk);font-style:normal;font-size:1.1rem;box-shadow:inset 0 0 0 3px var(--board-deep)}
nav.main{display:flex;gap:6px;flex-wrap:wrap}
nav.main a{text-decoration:none;font-weight:700;padding:8px 14px;border-radius:999px;color:var(--ink);background:var(--paper);border:2px solid var(--line)}
nav.main a.on,nav.main a:hover{background:var(--board);color:#fff;border-color:var(--board)}
h1,h2,h3{font-family:var(--display);line-height:1.2;text-wrap:balance;margin:0}
h1{font-size:clamp(1.9rem,5vw,2.8rem);font-weight:800}
h2{font-size:1.4rem;font-weight:700;margin-top:8px}
h3{font-size:1.1rem;font-weight:700}
.eyebrow{text-transform:uppercase;letter-spacing:.08em;font-size:.8rem;font-weight:800;color:var(--muted)}
.hero{background:var(--board);color:#fff;border-radius:22px;padding:28px 22px;margin-block:8px 20px;position:relative;overflow:hidden}
.hero::after{content:"";position:absolute;inset:auto -40px -60px auto;width:220px;height:220px;border-radius:50%;background:var(--board-deep);opacity:.7}
.hero>*{position:relative}
.hero .eyebrow{color:var(--chalk)}
.hero p{max-width:34em;font-size:1.05rem;margin:10px 0 18px}
.hero .by{color:#dfeee6;font-size:.95rem}
.levels{display:grid;grid-template-columns:repeat(3,1fr);gap:12px}
@media (max-width:620px){.levels{grid-template-columns:1fr}}
.level{display:flex;flex-direction:column;gap:6px;background:var(--paper);border:2px solid var(--line);border-top:8px solid var(--c);border-radius:16px;padding:16px;text-decoration:none;color:var(--ink);box-shadow:var(--shadow);transition:transform .08s;min-width:0}
.level:active{transform:translateY(3px);box-shadow:none}
.level .big{font-size:2.2rem;line-height:1}
.level b{font-family:var(--display);font-size:1.15rem}
.level small{color:var(--muted);font-weight:600;line-height:1.35}
.level .go{margin-top:auto;font-weight:800;color:var(--c)}
.level.soon{opacity:.75;border-style:dashed}
section{margin-block:28px}
.steps{display:grid;grid-template-columns:repeat(3,1fr);gap:12px;counter-reset:s}
@media (max-width:620px){.steps{grid-template-columns:1fr}}
.steps div{background:var(--paper);border:2px solid var(--line);border-radius:14px;padding:14px;font-weight:600;color:var(--muted);min-width:0}
.steps div::before{counter-increment:s;content:counter(s);display:inline-grid;place-items:center;width:30px;height:30px;border-radius:50%;background:var(--chalk);color:#1c2a24;font-family:var(--display);font-weight:800;margin-bottom:8px}
.steps b{display:block;color:var(--ink);font-family:var(--display)}
.promise{background:var(--paper);border:2px solid var(--line);border-radius:16px;padding:16px 18px}
.promise ul{margin:8px 0 0;padding-left:1.2em}
.promise li{margin:4px 0}
.posts{display:grid;gap:12px}
.post{display:block;background:var(--paper);border:2px solid var(--line);border-radius:14px;padding:14px 16px;text-decoration:none;color:var(--ink)}
.post b{font-family:var(--display);display:block;font-size:1.05rem}
.post small{color:var(--muted);font-weight:600}
.btn{display:inline-block;background:var(--chalk);color:#1c2a24;font-family:var(--display);font-weight:800;padding:12px 20px;border-radius:999px;text-decoration:none;box-shadow:0 4px 0 rgba(0,0,0,.2)}
.btn.ghost{background:var(--paper);color:var(--ink);border:2px solid var(--line);box-shadow:var(--shadow)}
article{max-width:680px}
article p,article li{font-size:1.05rem}
article h2{margin-top:28px}
article .meta{color:var(--muted);font-weight:600;font-size:.95rem;margin-bottom:18px}
article table{border-collapse:collapse;width:100%;font-size:.98rem}
article th,article td{border-bottom:1px solid var(--line);padding:8px 6px;text-align:left;vertical-align:top}
article th{font-family:var(--display);font-weight:700}
.tbl{overflow-x:auto}
.callout{background:var(--paper);border-left:6px solid var(--chalk);border-radius:0 12px 12px 0;padding:12px 16px;margin:16px 0}
footer.site{margin-top:40px;padding-block:20px 28px;border-top:2px solid var(--line);color:var(--muted);font-size:.9rem;font-weight:600;display:flex;justify-content:space-between;gap:12px;flex-wrap:wrap}