:root {
  --bg: #f6f7fb;
  --surface: #ffffff;
  --surface-2: #f1f3f8;
  --code-bg: #f7f8fa;
  --border: #e2e5ee;
  --text: #1d2433;
  --muted: #5d6679;
  --accent: #3b5bdb;
  --accent-soft: #e7ecff;
  --ok: #1f9d55;
  --ok-soft: #e3f6ea;
  --err: #d6336c;
  --err-soft: #fde8ef;
  --warn: #e8590c;
  --warn-soft: #fff1e6;
  --chip: #eef1f7;
  --kw: #a626a4;
  --str: #2f8a3c;
  --num: #b76b01;
  --com: #8a93a6;
  --fn: #2856c4;
  --shadow: 0 1px 2px rgba(20, 30, 60, .06), 0 4px 14px rgba(20, 30, 60, .05);
  --mono: "JetBrains Mono", "D2Coding", Consolas, monospace;
  color-scheme: light;
}
@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) {
    --bg: #12151c; --surface: #1a1e28; --surface-2: #222736; --code-bg: #151922;
    --border: #2d3343; --text: #e6e9f0; --muted: #9aa3b5; --accent: #7c95ff; --accent-soft: #252d4a;
    --ok: #4cc584; --ok-soft: #173326; --err: #ff6b9a; --err-soft: #3a1a26; --warn: #ff9a52; --warn-soft: #3a2616;
    --chip: #262b3a; --kw: #d69af0; --str: #8fd18f; --num: #f0b35a; --com: #7a8399; --fn: #8fb0ff;
    --shadow: 0 1px 2px rgba(0,0,0,.3); color-scheme: dark;
  }
}
:root[data-theme="dark"] {
  --bg: #12151c; --surface: #1a1e28; --surface-2: #222736; --code-bg: #151922;
  --border: #2d3343; --text: #e6e9f0; --muted: #9aa3b5; --accent: #7c95ff; --accent-soft: #252d4a;
  --ok: #4cc584; --ok-soft: #173326; --err: #ff6b9a; --err-soft: #3a1a26; --warn: #ff9a52; --warn-soft: #3a2616;
  --chip: #262b3a; --kw: #d69af0; --str: #8fd18f; --num: #f0b35a; --com: #7a8399; --fn: #8fb0ff;
  --shadow: 0 1px 2px rgba(0,0,0,.3); color-scheme: dark;
}

* { box-sizing: border-box; }
html, body { margin: 0; }
body {
  background: var(--bg); color: var(--text);
  font-family: Pretendard, -apple-system, "Apple SD Gothic Neo", "Malgun Gothic", sans-serif;
  font-size: 15.5px; line-height: 1.6; word-break: keep-all; overflow-wrap: anywhere;
}
button { font: inherit; color: inherit; cursor: pointer; }
code, pre, .mono { font-family: var(--mono); }
h1, h2, h3, h4 { line-height: 1.3; margin: 0; }

/* top bar */
.topbar {
  position: sticky; top: 0; z-index: 20; background: var(--surface);
  border-bottom: 1px solid var(--border); padding: 10px 20px;
  display: flex; align-items: center; justify-content: space-between; gap: 12px; flex-wrap: wrap;
}
.brand { display: flex; align-items: center; gap: 12px; min-width: 0; }
.logo { font-size: 28px; }
.brand h1 { font-size: 18px; }
.sub { margin: 0; font-size: 12.5px; color: var(--muted); }
.tabs { display: flex; gap: 4px; flex-wrap: wrap; }
.tab {
  border: 0; background: transparent; padding: 8px 14px; border-radius: 999px;
  color: var(--muted); font-weight: 600; font-size: 14.5px; position: relative;
}
.tab:hover { background: var(--surface-2); }
.tab.active { background: var(--accent); color: #fff; }
.badge {
  display: inline-block; min-width: 18px; padding: 0 6px; border-radius: 999px; font-size: 11.5px;
  background: var(--err); color: #fff; line-height: 18px; text-align: center; margin-left: 4px;
}
.progressbar { height: 4px; background: var(--border); position: sticky; top: 0; }
#progFill { height: 100%; width: 0; background: linear-gradient(90deg, var(--accent), var(--ok)); transition: width .4s; }

.view { display: none; }
.view.active { display: block; }
#view-practice.active { display: grid; grid-template-columns: 250px minmax(0, 1fr); gap: 24px; max-width: 1240px; margin: 0 auto; padding: 20px; }
.wrap { max-width: 980px; margin: 0 auto; padding: 24px 20px 40px; }

/* step navigation */
.steps { position: sticky; top: 86px; align-self: start; display: flex; flex-direction: column; gap: 4px; }
.steps h3 { font-size: 12.5px; color: var(--muted); font-weight: 600; margin: 4px 8px 6px; letter-spacing: .02em; }
.stepbtn {
  display: flex; align-items: center; gap: 10px; text-align: left; border: 1px solid transparent;
  background: transparent; padding: 8px 10px; border-radius: 10px; font-size: 14.5px;
}
.stepbtn:hover { background: var(--surface); border-color: var(--border); }
.stepbtn.active { background: var(--surface); border-color: var(--accent); box-shadow: var(--shadow); }
.stepnum {
  flex: none; width: 26px; height: 26px; border-radius: 50%; display: grid; place-items: center;
  background: var(--chip); font-size: 13px; font-weight: 700; color: var(--muted);
}
.stepbtn.done .stepnum { background: var(--ok); color: #fff; }
.stepbtn .meta { display: block; font-size: 12px; color: var(--muted); }

/* notebook */
.notebook { min-width: 0; display: flex; flex-direction: column; gap: 18px; padding-bottom: 60px; }
.step-hero {
  background: var(--surface); border: 1px solid var(--border); border-radius: 16px; padding: 22px 24px; box-shadow: var(--shadow);
}
.step-hero .kicker { font-size: 13px; font-weight: 700; color: var(--accent); }
.step-hero h2 { font-size: 24px; margin: 4px 0 8px; }
.step-hero p { margin: 6px 0; color: var(--text); }
.goal { background: var(--accent-soft); border-radius: 10px; padding: 10px 14px; margin-top: 12px; font-size: 14.5px; }

.cell {
  background: var(--surface); border: 1px solid var(--border); border-radius: 14px; box-shadow: var(--shadow); overflow: hidden;
}
.cell.done { border-color: color-mix(in srgb, var(--ok) 45%, var(--border)); }
.cell-head { padding: 14px 18px 4px; }
.cell-head h3 { font-size: 16.5px; display: flex; gap: 8px; align-items: baseline; flex-wrap: wrap; }
.cell-head h3 .no { color: var(--accent); font-weight: 800; }
.cell-head p { margin: 4px 0 6px; color: var(--muted); font-size: 14.5px; }
.cell-head .tip { font-size: 13.5px; background: var(--warn-soft); color: var(--text); border-radius: 8px; padding: 6px 10px; margin: 6px 0; }

.code-row { display: flex; gap: 10px; padding: 8px 18px 12px; align-items: flex-start; }
.runbtn {
  flex: none; width: 34px; height: 34px; border-radius: 50%; border: 0; background: var(--text); color: var(--surface);
  display: grid; place-items: center; font-size: 13px; margin-top: 6px; transition: transform .1s;
}
.runbtn:hover { transform: scale(1.08); background: var(--accent); color: #fff; }
.runbtn:disabled { opacity: .5; }
.exec { font-family: var(--mono); font-size: 12px; color: var(--muted); width: 34px; text-align: center; margin-top: 2px; }
.code {
  flex: 1; min-width: 0; background: var(--code-bg); border: 1px solid var(--border); border-radius: 8px;
  padding: 10px 12px; font-family: var(--mono); font-size: 14px; line-height: 1.75; white-space: pre-wrap; overflow-x: auto;
}
.code .kw { color: var(--kw); } .code .st { color: var(--str); } .code .nu { color: var(--num); }
.code .cm { color: var(--com); font-style: italic; } .code .fn { color: var(--fn); }
input.blank {
  font-family: var(--mono); font-size: 14px; padding: 1px 6px; border-radius: 5px; min-width: 4ch;
  border: 1.5px dashed var(--accent); background: var(--accent-soft); color: var(--text); outline: none;
}
input.blank:focus { border-style: solid; box-shadow: 0 0 0 3px color-mix(in srgb, var(--accent) 25%, transparent); }
input.blank.right { border: 1.5px solid var(--ok); background: var(--ok-soft); }
input.blank.wrong { border: 1.5px solid var(--err); background: var(--err-soft); animation: shake .3s; }
input.blank.revealed { border: 1.5px solid var(--warn); background: var(--warn-soft); }
@keyframes shake { 25% { transform: translateX(-3px); } 75% { transform: translateX(3px); } }

.cell-tools { display: flex; gap: 6px; flex-wrap: wrap; padding: 0 18px 12px 62px; }
.chipbtn {
  border: 1px solid var(--border); background: var(--surface); border-radius: 999px; padding: 4px 12px; font-size: 13px; color: var(--muted);
}
.chipbtn:hover { color: var(--text); border-color: var(--muted); }
.chipbtn.primary { background: var(--accent); color: #fff; border-color: var(--accent); }
.hintbox { margin: 0 18px 12px 62px; padding: 8px 12px; border-radius: 8px; background: var(--warn-soft); font-size: 14px; }

.output { border-top: 1px dashed var(--border); padding: 12px 18px 16px 62px; display: none; }
.output.show { display: block; animation: fade .3s; }
@keyframes fade { from { opacity: 0; transform: translateY(4px); } }
.output pre { margin: 0; font-size: 12.5px; line-height: 1.5; white-space: pre; overflow-x: auto; background: transparent; }
.output .err { color: var(--err); background: var(--err-soft); border-radius: 8px; padding: 10px 12px; white-space: pre-wrap; }
.output .err b { display: block; }
.output .ok-line { color: var(--ok); font-size: 13.5px; font-weight: 600; }
.err-help { font-family: Pretendard, sans-serif; color: var(--text); font-size: 14px; margin-top: 6px; }

.dftable { overflow-x: auto; border: 1px solid var(--border); border-radius: 8px; }
.dftable table { border-collapse: collapse; font-size: 12.5px; font-family: var(--mono); }
.dftable th, .dftable td { padding: 4px 10px; text-align: right; white-space: nowrap; border-bottom: 1px solid var(--border); }
.dftable thead th { background: var(--surface-2); position: sticky; top: 0; }
.dftable tbody th { color: var(--muted); }
.dftable tbody tr:nth-child(even) { background: var(--surface-2); }
.shape-note { font-size: 12.5px; color: var(--muted); margin-top: 4px; font-family: var(--mono); }

.interp { margin-top: 12px; border-left: 4px solid var(--accent); background: var(--accent-soft); padding: 8px 12px; border-radius: 0 8px 8px 0; font-size: 14.5px; }
.interp b { color: var(--accent); }
.interp ul { margin: 4px 0 0; padding-left: 20px; }
.simnote { font-size: 12.5px; color: var(--muted); margin-top: 6px; }

/* charts */
.chart { max-width: 100%; overflow-x: auto; }
.chart svg { display: block; max-width: 100%; height: auto; font-family: Pretendard, sans-serif; }
.chart .ax { stroke: var(--muted); stroke-width: 1; }
.chart .grid { stroke: var(--border); stroke-width: 1; }
.chart text { fill: var(--muted); font-size: 11px; }
.chart text.ttl { fill: var(--text); font-size: 12.5px; font-weight: 600; }
.legend { display: flex; gap: 14px; flex-wrap: wrap; font-size: 13px; color: var(--muted); margin-top: 4px; }
.legend i { display: inline-block; width: 11px; height: 11px; border-radius: 3px; margin-right: 5px; vertical-align: -1px; }
.hm-info { font-size: 13.5px; min-height: 1.6em; margin-top: 6px; }

/* explore panels */
.panel { background: var(--surface); border: 1px solid var(--border); border-radius: 14px; padding: 18px; box-shadow: var(--shadow); }
.panel h3 { font-size: 16.5px; margin-bottom: 4px; }
.panel > p { margin: 2px 0 10px; color: var(--muted); font-size: 14.5px; }
.panel.tryit { border: 1.5px solid var(--accent); }
.tag { display: inline-block; font-size: 12px; font-weight: 700; padding: 1px 8px; border-radius: 999px; background: var(--accent); color: #fff; margin-right: 6px; vertical-align: 2px; }
.ctrl { display: flex; gap: 10px; align-items: center; flex-wrap: wrap; margin: 6px 0 10px; }
select, .ctrl input[type=range] { font: inherit; }
select { padding: 6px 10px; border-radius: 8px; border: 1px solid var(--border); background: var(--surface); color: var(--text); }
input[type=range] { accent-color: var(--accent); width: min(320px, 100%); }
.big { font-size: 26px; font-weight: 800; color: var(--accent); }
.splitbar { display: flex; height: 34px; border-radius: 8px; overflow: hidden; font-size: 13px; font-weight: 700; color: #fff; }
.splitbar div { display: grid; place-items: center; transition: width .25s; white-space: nowrap; overflow: hidden; }

/* concept quiz in steps */
.check { margin-top: 10px; }
.check .q { font-weight: 600; margin-bottom: 6px; }
.opts { display: flex; flex-wrap: wrap; gap: 8px; }
.opt {
  border: 1.5px solid var(--border); background: var(--surface); border-radius: 10px; padding: 8px 14px; font-size: 14.5px; text-align: left;
}
.opt:hover { border-color: var(--accent); }
.opt.right { border-color: var(--ok); background: var(--ok-soft); }
.opt.wrong { border-color: var(--err); background: var(--err-soft); }
.feedback { margin-top: 8px; font-size: 14.5px; }
.feedback.good { color: var(--ok); } .feedback.bad { color: var(--err); }

.featgrid { display: grid; grid-template-columns: repeat(auto-fill, minmax(230px, 1fr)); gap: 6px; margin-top: 10px; font-size: 13.5px; }
.featgrid div { background: var(--surface-2); border-radius: 8px; padding: 6px 10px; }
.featgrid code { font-weight: 600; font-size: 12.5px; }
.featgrid .target { background: var(--accent-soft); outline: 1.5px solid var(--accent); }

/* file panel (colab mock) */
.files { display: grid; grid-template-columns: 1fr auto 1fr; gap: 12px; align-items: center; margin: 4px 18px 12px 62px; }
.drop {
  border: 2px dashed var(--border); border-radius: 12px; padding: 12px; min-height: 86px; font-size: 13.5px; color: var(--muted);
}
.drop.over { border-color: var(--accent); background: var(--accent-soft); }
.drop h4 { font-size: 13px; margin-bottom: 6px; color: var(--text); }
.file {
  display: inline-flex; align-items: center; gap: 6px; background: var(--surface-2); border: 1px solid var(--border);
  border-radius: 8px; padding: 6px 10px; font-family: var(--mono); font-size: 12.5px; cursor: grab; color: var(--text);
}
.arrow { font-size: 20px; color: var(--muted); }

.next-step { display: flex; justify-content: space-between; gap: 10px; flex-wrap: wrap; }
.navbtn { border: 1px solid var(--border); background: var(--surface); padding: 10px 18px; border-radius: 10px; font-weight: 600; }
.navbtn.primary { background: var(--accent); color: #fff; border-color: var(--accent); }
.navbtn:disabled { opacity: .4; cursor: default; }

/* quiz */
.view-head { margin-bottom: 18px; }
.view-head h2 { font-size: 24px; margin-bottom: 4px; }
.view-head p { margin: 0; color: var(--muted); }
.qsetup { display: grid; grid-template-columns: repeat(auto-fit, minmax(240px, 1fr)); gap: 12px; }
.mode {
  background: var(--surface); border: 1.5px solid var(--border); border-radius: 14px; padding: 16px; text-align: left; box-shadow: var(--shadow);
}
.mode:hover { border-color: var(--accent); }
.mode .ic { font-size: 26px; }
.mode h3 { font-size: 16.5px; margin: 6px 0 2px; }
.mode p { margin: 0; color: var(--muted); font-size: 14px; }
.qcard { background: var(--surface); border: 1px solid var(--border); border-radius: 16px; padding: 22px; box-shadow: var(--shadow); }
.qtop { display: flex; justify-content: space-between; align-items: center; color: var(--muted); font-size: 13.5px; margin-bottom: 10px; gap: 8px; flex-wrap: wrap; }
.qdots { display: flex; gap: 4px; flex-wrap: wrap; }
.qdots i { width: 10px; height: 10px; border-radius: 50%; background: var(--border); }
.qdots i.cur { background: var(--accent); } .qdots i.o { background: var(--ok); } .qdots i.x { background: var(--err); }
.qtext { font-size: 18px; font-weight: 700; margin-bottom: 12px; }
.qcode { background: var(--code-bg); border: 1px solid var(--border); border-radius: 8px; padding: 10px 12px; font-family: var(--mono); font-size: 14px; white-space: pre-wrap; margin-bottom: 12px; line-height: 1.75; }
.order-list { display: flex; flex-direction: column; gap: 6px; }
.order-item {
  display: flex; align-items: center; gap: 10px; background: var(--surface-2); border: 1.5px solid var(--border);
  border-radius: 10px; padding: 8px 12px; font-size: 14.5px;
}
.order-item .mv { margin-left: auto; display: flex; gap: 4px; }
.order-item .mv button { border: 1px solid var(--border); background: var(--surface); border-radius: 6px; width: 30px; height: 28px; }
.order-item.right { border-color: var(--ok); background: var(--ok-soft); }
.order-item.wrong { border-color: var(--err); background: var(--err-soft); }
.order-item code { font-size: 13px; }
.explain { margin-top: 12px; padding: 10px 14px; border-radius: 10px; background: var(--surface-2); font-size: 14.5px; }
.qnav { display: flex; justify-content: flex-end; margin-top: 14px; gap: 8px; }
.result { text-align: center; }
.score { font-size: 54px; font-weight: 800; color: var(--accent); line-height: 1.1; }
.bars { display: grid; gap: 6px; margin: 16px auto; max-width: 520px; text-align: left; }
.bars .row { display: grid; grid-template-columns: 130px 1fr 52px; gap: 8px; align-items: center; font-size: 13.5px; }
.bars .track { height: 10px; background: var(--border); border-radius: 999px; overflow: hidden; }
.bars .fill { height: 100%; background: var(--accent); }

/* cards */
.cardgroup { margin-bottom: 22px; }
.cardgroup h3 { font-size: 15.5px; margin-bottom: 8px; color: var(--muted); }
.cards { display: grid; grid-template-columns: repeat(auto-fill, minmax(260px, 1fr)); gap: 10px; }
.ccard { background: var(--surface); border: 1px solid var(--border); border-radius: 12px; padding: 12px 14px; text-align: left; box-shadow: var(--shadow); }
.ccard code { font-size: 13.5px; font-weight: 600; color: var(--fn); display: block; margin-bottom: 4px; }
.ccard .mean { font-size: 14px; }
.ccard .ex { font-family: var(--mono); font-size: 12px; color: var(--muted); margin-top: 6px; white-space: pre-wrap; }
.hide .ccard:not(.open) .mean, .hide .ccard:not(.open) .ex { filter: blur(6px); user-select: none; }
.switch { display: inline-flex; gap: 6px; align-items: center; margin-top: 10px; font-size: 14px; font-weight: 600; }

/* notes */
.note { background: var(--surface); border: 1px solid var(--border); border-left: 4px solid var(--err); border-radius: 10px; padding: 12px 16px; margin-bottom: 10px; }
.note .src { font-size: 12.5px; color: var(--muted); }
.note .ans { font-family: var(--mono); color: var(--ok); font-weight: 600; }
.empty { text-align: center; color: var(--muted); padding: 40px 10px; }

.foot { text-align: center; color: var(--muted); font-size: 12.5px; padding: 20px 16px 30px; }
.link { border: 0; background: none; color: var(--accent); text-decoration: underline; font-size: 12.5px; margin-left: 8px; }
#toast {
  position: fixed; left: 50%; bottom: 24px; transform: translateX(-50%) translateY(20px); opacity: 0; pointer-events: none;
  background: var(--text); color: var(--surface); padding: 10px 18px; border-radius: 999px; font-weight: 600; transition: .25s; z-index: 50; max-width: calc(100% - 32px);
}
#toast.show { opacity: 1; transform: translateX(-50%); }

@media (max-width: 860px) {
  #view-practice.active { grid-template-columns: minmax(0, 1fr); padding: 12px 16px; gap: 12px; }
  .steps { position: static; flex-direction: row; overflow-x: auto; padding-bottom: 4px; min-width: 0; }
  .topbar { position: static; }
  .brand, .tabs { width: 100%; min-width: 0; }
  .brand > div { min-width: 0; }
  .sub { white-space: nowrap; overflow: hidden; text-overflow: ellipsis; }
  .tabs { flex-wrap: nowrap; overflow-x: auto; }
  .tab { flex: none; padding: 7px 12px; font-size: 14px; }
  .opts { flex-direction: column; align-items: stretch; }
  .steps h3 { display: none; }
  .stepbtn { flex: none; padding: 6px 10px; }
  .stepbtn .meta { display: none; }
  .topbar { padding: 10px 16px; }
  .wrap { padding: 16px 16px 30px; }
  .cell-tools, .hintbox, .output, .files { margin-left: 0; padding-left: 18px; }
  .hintbox { margin-left: 18px; }
  .files { grid-template-columns: 1fr; margin-left: 18px; margin-right: 18px; padding-left: 0; }
  .arrow { transform: rotate(90deg); justify-self: center; }
  .code-row { padding: 8px 12px 12px; }
  .step-hero { padding: 18px; }
  .bars .row { grid-template-columns: 96px 1fr 44px; }
}