:root {
  --bg: #0f1420;
  --panel: #171d2e;
  --panel-2: #1e2740;
  --ink: #e8ecf5;
  --muted: #96a0bd;
  --accent: #5aa9ff;
  --accent-2: #ff7a9c;
  --line: #2b3550;
  --grid: #222a40;
  --axis: #6b7699;
}

* { box-sizing: border-box; }

body {
  margin: 0;
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
  background: radial-gradient(1200px 600px at 70% -10%, #1a2136, var(--bg));
  color: var(--ink);
  min-height: 100vh;
}

header {
  padding: 28px 24px 12px;
  text-align: center;
}
header h1 { margin: 0; font-size: 28px; letter-spacing: 0.3px; }
.tagline { margin: 6px 0 0; color: var(--muted); }

.concepts {
  display: flex;
  gap: 10px;
  justify-content: center;
  padding: 12px;
  flex-wrap: wrap;
}
.concept-btn {
  background: var(--panel);
  color: var(--ink);
  border: 1px solid var(--line);
  padding: 8px 16px;
  border-radius: 999px;
  cursor: pointer;
  font-size: 14px;
  transition: background 0.15s, border-color 0.15s;
}
.concept-btn.active { border-color: var(--accent); background: var(--panel-2); }

main { max-width: 980px; margin: 0 auto; padding: 8px 16px 32px; }

.panel { display: none; }
.panel.active { display: block; }

.controls {
  display: flex;
  flex-wrap: wrap;
  gap: 22px;
  background: var(--panel);
  border: 1px solid var(--line);
  border-radius: 14px;
  padding: 16px 18px;
  margin-bottom: 16px;
}
.control-group { display: flex; flex-direction: column; gap: 10px; }
.control-group .label { color: var(--muted); font-size: 12px; text-transform: uppercase; letter-spacing: 1px; }

.toggle { display: inline-flex; background: var(--panel-2); border-radius: 10px; padding: 4px; gap: 4px; }
.fn-btn {
  background: transparent;
  color: var(--muted);
  border: none;
  padding: 8px 14px;
  border-radius: 8px;
  cursor: pointer;
  font-size: 13px;
}
.fn-btn.active { background: var(--accent); color: #06101f; font-weight: 600; }

.control-group label { font-size: 14px; color: var(--ink); display: flex; align-items: center; gap: 8px; }
.slider-group input[type="range"] { width: 200px; accent-color: var(--accent); }

canvas {
  width: 100%;
  height: auto;
  background: var(--panel);
  border: 1px solid var(--line);
  border-radius: 14px;
  display: block;
  cursor: crosshair;
}

.readout { margin-top: 14px; }
.formula {
  font-family: "SF Mono", ui-monospace, Menlo, Consolas, monospace;
  font-size: 18px;
  color: var(--accent);
  margin: 0 0 6px;
}
.hint { color: var(--muted); font-size: 13px; margin: 0; }

footer { text-align: center; color: var(--muted); padding: 20px; font-size: 13px; }