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

:root {
  --bg: #0f1729;
  --surface: #1a2340;
  --surface-hover: #22305a;
  --border: #2a3a6a;
  --text: #e8ecf4;
  --text-muted: #8892a8;
  --accent: #6c5ce7;
  --accent-hover: #7d6ff0;
  --accent-glow: rgba(108, 92, 231, 0.25);
  --result-bg: #162040;
  --radius: 12px;
  --radius-sm: 8px;
}

html {
  font-size: 16px;
}

body {
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
  background: var(--bg);
  color: var(--text);
  min-height: 100vh;
  line-height: 1.5;
  -webkit-font-smoothing: antialiased;
}

.sr-only {
  position: absolute;
  width: 1px;
  height: 1px;
  padding: 0;
  margin: -1px;
  overflow: hidden;
  clip: rect(0, 0, 0, 0);
  white-space: nowrap;
  border: 0;
}

.app {
  max-width: 540px;
  margin: 0 auto;
  padding: 2rem 1rem;
  min-height: 100vh;
  display: flex;
  flex-direction: column;
}

.header {
  text-align: center;
  margin-bottom: 2rem;
}

.header h1 {
  font-size: 2rem;
  font-weight: 700;
  letter-spacing: -0.02em;
  background: linear-gradient(135deg, var(--accent), #a29bfe);
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
  background-clip: text;
}

.subtitle {
  color: var(--text-muted);
  font-size: 1rem;
  margin-top: 0.25rem;
}

.converter {
  flex: 1;
}

.category-tabs {
  display: flex;
  gap: 0.25rem;
  background: var(--surface);
  padding: 4px;
  border-radius: var(--radius);
  margin-bottom: 1.5rem;
}

.tab {
  flex: 1;
  padding: 0.625rem 0.5rem;
  border: none;
  background: transparent;
  color: var(--text-muted);
  font-size: 0.875rem;
  font-weight: 500;
  cursor: pointer;
  border-radius: var(--radius-sm);
  transition: all 0.2s ease;
}

.tab:hover {
  color: var(--text);
}

.tab.active {
  background: var(--accent);
  color: #fff;
  box-shadow: 0 2px 8px var(--accent-glow);
}

.conversion-panel {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 0.75rem;
}

.input-group {
  width: 100%;
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 1rem;
  display: flex;
  gap: 0.75rem;
  align-items: center;
  transition: border-color 0.2s ease;
}

.input-group:focus-within {
  border-color: var(--accent);
  box-shadow: 0 0 0 3px var(--accent-glow);
}

.value-input {
  flex: 1;
  background: transparent;
  border: none;
  color: var(--text);
  font-size: 1.5rem;
  font-weight: 600;
  outline: none;
  min-width: 0;
  font-family: inherit;
}

.value-input::placeholder {
  color: var(--text-muted);
  font-weight: 400;
}

.value-input.result {
  color: #a29bfe;
}

.unit-select {
  background: var(--surface-hover);
  border: 1px solid var(--border);
  color: var(--text);
  padding: 0.5rem 0.75rem;
  border-radius: var(--radius-sm);
  font-size: 0.875rem;
  font-weight: 500;
  cursor: pointer;
  outline: none;
  font-family: inherit;
  min-width: 100px;
}

.unit-select:focus {
  border-color: var(--accent);
}

.swap-btn {
  background: var(--surface);
  border: 1px solid var(--border);
  color: var(--text-muted);
  width: 44px;
  height: 44px;
  border-radius: 50%;
  display: flex;
  align-items: center;
  justify-content: center;
  cursor: pointer;
  transition: all 0.2s ease;
  flex-shrink: 0;
}

.swap-btn:hover {
  color: var(--accent);
  border-color: var(--accent);
  background: var(--surface-hover);
}

.swap-btn:active {
  transform: rotate(180deg);
}

.formula {
  margin-top: 1.25rem;
  padding: 0.875rem 1rem;
  background: var(--result-bg);
  border-radius: var(--radius-sm);
  border-left: 3px solid var(--accent);
}

.formula p {
  color: var(--text-muted);
  font-size: 0.875rem;
  font-family: "SF Mono", "Fira Code", "Cascadia Code", monospace;
}

.quick-refs {
  margin-top: 2rem;
}

.quick-refs h2 {
  font-size: 1rem;
  font-weight: 600;
  color: var(--text-muted);
  margin-bottom: 0.75rem;
  text-transform: uppercase;
  letter-spacing: 0.05em;
}

.ref-cards {
  display: grid;
  grid-template-columns: repeat(2, 1fr);
  gap: 0.5rem;
}

.ref-card {
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: var(--radius-sm);
  padding: 0.75rem;
  transition: all 0.15s ease;
}

.ref-card:hover {
  border-color: var(--accent);
  background: var(--surface-hover);
}

.ref-card .ref-value {
  font-size: 0.8125rem;
  color: var(--text);
  font-weight: 500;
}

.ref-card .ref-label {
  font-size: 0.6875rem;
  color: var(--text-muted);
  margin-top: 0.125rem;
}

.footer {
  text-align: center;
  margin-top: 2.5rem;
  padding-top: 1.5rem;
  border-top: 1px solid var(--border);
}

.footer p {
  color: var(--text-muted);
  font-size: 0.8125rem;
}

@media (max-width: 480px) {
  .app {
    padding: 1.25rem 0.75rem;
  }

  .header h1 {
    font-size: 1.5rem;
  }

  .value-input {
    font-size: 1.25rem;
  }

  .category-tabs {
    flex-wrap: wrap;
  }

  .tab {
    flex-basis: calc(50% - 0.125rem);
  }

  .ref-cards {
    grid-template-columns: 1fr;
  }
}