:root {
  --bg: #fafaf7;
  --bg-card: #ffffff;
  --bg-elevated: #f4f3ee;
  --border: #e8e6dd;
  --border-strong: #d4d1c4;
  --text: #141b2d;
  --text-muted: #5a6478;
  --text-subtle: #8a94a6;
  --accent: #0b4f6c;
  --accent-light: #145b7a;
  --teal: #2a9d8f;
  --amber: #c2410c;
  --amber-light: #ea580c;
  --green: #4d7a5c;
  --depth: #0b4f6c;
  --surface: #86b0c2;
  --shadow-sm: 0 1px 2px rgba(11, 79, 108, 0.04);
  --shadow: 0 1px 3px rgba(11, 79, 108, 0.08), 0 1px 2px rgba(11, 79, 108, 0.04);
  --shadow-lg: 0 10px 30px rgba(11, 79, 108, 0.10), 0 4px 8px rgba(11, 79, 108, 0.04);
}

[data-theme="dark"] {
  --bg: #0a1220;
  --bg-card: #121d30;
  --bg-elevated: #18253a;
  --border: #1f3049;
  --border-strong: #2b4163;
  --text: #e8ecf1;
  --text-muted: #9aa5b8;
  --text-subtle: #6a7689;
  --accent: #5ec4e0;
  --accent-light: #7dd2ea;
  --teal: #5cd4c6;
  --amber: #fb923c;
  --amber-light: #fdba74;
  --green: #86c598;
  --depth: #5ec4e0;
  --surface: #6a95a8;
  --shadow-sm: 0 1px 2px rgba(0, 0, 0, 0.3);
  --shadow: 0 1px 3px rgba(0, 0, 0, 0.4), 0 1px 2px rgba(0, 0, 0, 0.2);
  --shadow-lg: 0 10px 30px rgba(0, 0, 0, 0.5), 0 4px 8px rgba(0, 0, 0, 0.3);
}

*, *::before, *::after {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

html { scroll-behavior: smooth; }

body {
  font-family: 'IBM Plex Serif', Georgia, serif;
  background: var(--bg);
  color: var(--text);
  line-height: 1.65;
  font-size: 16px;
  -webkit-font-smoothing: antialiased;
  transition: background 0.25s ease, color 0.25s ease;
}

.mono { font-family: 'IBM Plex Mono', monospace; }

.container {
  max-width: 720px;
  margin: 0 auto;
  padding: 2.5rem 1.25rem 6rem;
}

/* Header */
header {
  display: flex;
  justify-content: space-between;
  align-items: flex-start;
  gap: 1rem;
  padding-bottom: 2rem;
  margin-bottom: 2.5rem;
  border-bottom: 1px solid var(--border);
}

.brand-block { min-width: 0; }

.brand {
  font-size: 1.625rem;
  font-weight: 500;
  letter-spacing: -0.02em;
  color: var(--text);
  line-height: 1.1;
}

.brand span { color: var(--accent); }

.tagline {
  font-family: 'IBM Plex Sans', sans-serif;
  font-size: 0.8125rem;
  color: var(--text-muted);
  margin-top: 0.5rem;
  letter-spacing: 0.01em;
}

.theme-toggle {
  font-family: 'IBM Plex Sans', sans-serif;
  font-size: 0.6875rem;
  font-weight: 500;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  color: var(--text-muted);
  background: transparent;
  border: 1px solid var(--border-strong);
  border-radius: 3px;
  padding: 0.5rem 0.75rem;
  cursor: pointer;
  transition: all 0.15s ease;
  white-space: nowrap;
  flex-shrink: 0;
}

.theme-toggle:hover {
  border-color: var(--accent);
  color: var(--accent);
}

/* Section labels */
.section-label {
  font-family: 'IBM Plex Sans', sans-serif;
  font-size: 0.6875rem;
  font-weight: 600;
  letter-spacing: 0.12em;
  text-transform: uppercase;
  color: var(--text-subtle);
  margin-bottom: 0.625rem;
}

/* Inputs card */
.inputs {
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-radius: 6px;
  padding: 1.75rem;
  margin-bottom: 2.5rem;
  box-shadow: var(--shadow);
}

.input-group { margin-bottom: 1.375rem; }
.input-group:last-child { margin-bottom: 0; }

label {
  display: block;
  font-family: 'IBM Plex Sans', sans-serif;
  font-size: 0.8125rem;
  font-weight: 500;
  color: var(--text);
  margin-bottom: 0.5rem;
}

.toggle-group {
  display: inline-flex;
  background: var(--bg-elevated);
  border: 1px solid var(--border);
  border-radius: 4px;
  padding: 2px;
  width: 100%;
  max-width: 320px;
}

.toggle-btn {
  flex: 1;
  font-family: 'IBM Plex Sans', sans-serif;
  font-size: 0.8125rem;
  font-weight: 500;
  padding: 0.5rem 1rem;
  background: transparent;
  border: none;
  color: var(--text-muted);
  cursor: pointer;
  border-radius: 3px;
  transition: all 0.15s ease;
}

.toggle-btn.active {
  background: var(--bg-card);
  color: var(--accent);
  box-shadow: var(--shadow-sm);
}

.toggle-btn:not(.active):hover { color: var(--text); }

input[type="number"], select {
  width: 100%;
  font-family: 'IBM Plex Sans', sans-serif;
  font-size: 0.9375rem;
  padding: 0.625rem 0.875rem;
  background: var(--bg-card);
  border: 1px solid var(--border-strong);
  border-radius: 4px;
  color: var(--text);
  transition: border-color 0.15s ease;
  -webkit-appearance: none;
  -moz-appearance: textfield;
}

input[type="number"]::-webkit-outer-spin-button,
input[type="number"]::-webkit-inner-spin-button {
  -webkit-appearance: none;
  margin: 0;
}

input[type="number"]:focus, select:focus {
  outline: none;
  border-color: var(--accent);
  box-shadow: 0 0 0 3px rgba(11, 79, 108, 0.08);
}

[data-theme="dark"] input[type="number"]:focus,
[data-theme="dark"] select:focus {
  box-shadow: 0 0 0 3px rgba(94, 196, 224, 0.15);
}

.input-with-unit {
  position: relative;
  max-width: 220px;
}

.input-with-unit .unit-suffix {
  position: absolute;
  right: 0.875rem;
  top: 50%;
  transform: translateY(-50%);
  font-family: 'IBM Plex Sans', sans-serif;
  font-size: 0.8125rem;
  color: var(--text-subtle);
  pointer-events: none;
}

.input-with-unit input { padding-right: 2.5rem; }

select {
  background-image: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='12' height='8' viewBox='0 0 12 8'%3E%3Cpath d='M1 1l5 5 5-5' stroke='%238a94a6' stroke-width='1.5' fill='none' stroke-linecap='round' stroke-linejoin='round'/%3E%3C/svg%3E");
  background-repeat: no-repeat;
  background-position: right 0.875rem center;
  padding-right: 2.25rem;
  max-width: 280px;
}

/* Result banner */
.result {
  background: linear-gradient(135deg, var(--accent) 0%, var(--accent-light) 100%);
  color: white;
  border-radius: 6px;
  padding: 2.25rem 2rem;
  margin-bottom: 1.25rem;
  text-align: center;
  box-shadow: var(--shadow-lg);
  position: relative;
  overflow: hidden;
}

.result::before {
  content: '';
  position: absolute;
  top: -50%;
  right: -20%;
  width: 300px;
  height: 300px;
  border-radius: 50%;
  background: radial-gradient(circle, rgba(255,255,255,0.08) 0%, transparent 70%);
  pointer-events: none;
}

.result-label {
  font-family: 'IBM Plex Sans', sans-serif;
  font-size: 0.6875rem;
  font-weight: 600;
  letter-spacing: 0.18em;
  text-transform: uppercase;
  opacity: 0.75;
  margin-bottom: 0.75rem;
}

.result-value {
  font-size: 4rem;
  font-weight: 300;
  letter-spacing: -0.03em;
  font-variant-numeric: tabular-nums;
  line-height: 1;
  display: flex;
  align-items: baseline;
  justify-content: center;
  gap: 0.5rem;
}

.result-value .unit {
  font-size: 1.5rem;
  font-weight: 400;
  opacity: 0.75;
  letter-spacing: 0;
}

.result-raw {
  font-family: 'IBM Plex Sans', sans-serif;
  font-size: 0.8125rem;
  opacity: 0.7;
  margin-top: 1rem;
  font-feature-settings: "tnum";
}

[data-theme="dark"] .result {
  color: var(--bg);
}

/* Section headings */
h2 {
  font-size: 1.75rem;
  font-weight: 400;
  letter-spacing: -0.015em;
  color: var(--text);
  margin-bottom: 0.5rem;
}

/* Summary / overview section */
.summary-section {
  margin-bottom: 4rem;
}

.summary-profile {
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-radius: 6px;
  padding: 1.5rem;
  margin-bottom: 1.25rem;
  box-shadow: var(--shadow);
}

.summary-profile h3 {
  font-family: 'IBM Plex Sans', sans-serif;
  font-size: 0.875rem;
  font-weight: 500;
  color: var(--text);
  margin-bottom: 1rem;
  letter-spacing: -0.005em;
}

.summary-profile svg {
  display: block;
  width: 100%;
  height: auto;
  max-height: 280px;
}

.summary-cat {
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-radius: 6px;
  padding: 1.25rem 1rem 1rem;
  margin-bottom: 1.25rem;
  box-shadow: var(--shadow);
}

.summary-cat svg {
  display: block;
  width: 100%;
  height: auto;
  max-height: 200px;
}

.summary-math {
  display: flex;
  flex-direction: column;
  gap: 0.625rem;
}

.summary-math .math { margin: 0; }

@media (max-width: 640px) {
  .summary-section { margin-bottom: 3rem; }
  .summary-profile { padding: 1.125rem; }
}

/* Steps */
.step {
  margin-top: 3.5rem;
  padding-top: 2.5rem;
  border-top: 1px solid var(--border);
}

.step:first-of-type {
  margin-top: 2rem;
  padding-top: 0;
  border-top: none;
}

.step-number {
  font-family: 'IBM Plex Mono', monospace;
  font-size: 0.75rem;
  font-weight: 500;
  color: var(--accent);
  letter-spacing: 0.1em;
  margin-bottom: 0.625rem;
  text-transform: uppercase;
}

.step h3 {
  font-size: 1.375rem;
  font-weight: 500;
  color: var(--text);
  margin-bottom: 0.875rem;
  letter-spacing: -0.015em;
}

.step p {
  font-size: 0.9375rem;
  color: var(--text-muted);
  line-height: 1.75;
  margin-bottom: 1.125rem;
  max-width: 60ch;
}

.step p strong { color: var(--text); font-weight: 600; }

/* Math display */
.math {
  font-family: 'IBM Plex Mono', monospace;
  font-size: 0.875rem;
  background: var(--bg-elevated);
  border-left: 3px solid var(--accent);
  padding: 1rem 1.25rem;
  margin: 1.25rem 0;
  border-radius: 0 4px 4px 0;
  overflow-x: auto;
  color: var(--text);
  line-height: 1.8;
}

.math .formula {
  color: var(--text-muted);
  font-size: 0.8125rem;
  margin-bottom: 0.375rem;
}

.math .compute { color: var(--text); }

.math .compute + .compute {
  padding-left: 1.25rem;
  position: relative;
}

.math .compute + .compute::before {
  content: '=';
  position: absolute;
  left: 0;
  color: var(--text-subtle);
}

.math .result-inline {
  color: var(--accent);
  font-weight: 600;
}

.math .cancel {
  text-decoration: line-through;
  text-decoration-color: var(--amber);
  text-decoration-thickness: 1.5px;
  opacity: 0.55;
}

.math sub {
  font-size: 0.7em;
  vertical-align: sub;
}

/* SVG visuals */
.visual {
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-radius: 6px;
  padding: 1.5rem 1.25rem 1rem;
  margin: 1.5rem 0;
}

.visual svg {
  display: block;
  width: 100%;
  height: auto;
  max-height: 280px;
}

.visual-caption {
  font-family: 'IBM Plex Sans', sans-serif;
  font-size: 0.75rem;
  color: var(--text-subtle);
  text-align: center;
  margin-top: 0.875rem;
  font-style: italic;
  line-height: 1.5;
}

/* SVG styling via classes */
svg text {
  fill: var(--text);
  font-family: 'IBM Plex Sans', sans-serif;
}

svg text.box-label { fill: #fff; }
svg .muted-text { fill: var(--text-muted); }
svg .subtle-text { fill: var(--text-subtle); }
svg .depth-fill { fill: var(--depth); }
svg .depth-stroke { stroke: var(--depth); }
svg .surface-fill { fill: var(--surface); }
svg .amber-fill { fill: var(--amber); }
svg .amber-stroke { stroke: var(--amber); }
svg .teal-fill { fill: var(--teal); }
svg .teal-stroke { stroke: var(--teal); }
svg .green-fill { fill: var(--green); }
svg .green-stroke { stroke: var(--green); }
svg .border-stroke { stroke: var(--border-strong); }
svg .bg-fill { fill: var(--bg-elevated); }

/* Footer */
footer {
  margin-top: 6rem;
  padding-top: 2.5rem;
  border-top: 1px solid var(--border);
  text-align: center;
  font-size: 0.8125rem;
  color: var(--text-subtle);
  font-family: 'IBM Plex Sans', sans-serif;
  line-height: 1.7;
}

footer p { margin-bottom: 0.375rem; }
footer .footer-credit {
  font-size: 0.6875rem;
  color: var(--text-subtle);
  opacity: 0.75;
  margin-top: 1rem;
  margin-bottom: 0.125rem;
}
footer a { color: var(--text-muted); text-decoration: underline; text-decoration-thickness: 1px; text-underline-offset: 2px; }

/* Responsive */
@media (max-width: 640px) {
  .container { padding: 2rem 1rem 4rem; }
  .brand { font-size: 1.375rem; }
  h2 { font-size: 1.5rem; }
  .result-value { font-size: 3rem; }
  .result-value .unit { font-size: 1.25rem; }
  .result { padding: 1.75rem 1.25rem; }
  .inputs { padding: 1.25rem; }
  .step h3 { font-size: 1.25rem; }
  .math { font-size: 0.8125rem; padding: 0.875rem 1rem; }
  .visual { padding: 1.125rem 0.875rem 0.75rem; }
}

/* Subtle entry animation */
@keyframes fadeIn {
  from { opacity: 0; transform: translateY(4px); }
  to { opacity: 1; transform: translateY(0); }
}

.inputs, .result { animation: fadeIn 0.35s ease-out both; }

/* Transitions for live updates */
.result-value, .math, .visual svg {
  transition: opacity 0.1s ease;
}