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

:root {
  --bg: #0f1117;
  --surface: #1a1d27;
  --border: #2e3345;
  --text: #e8eaed;
  --muted: #9aa0b0;
  --accent: #6c63ff;
}

body {
  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
  background: var(--bg);
  color: var(--text);
  line-height: 1.6;
  min-height: 100vh;
}

.container {
  max-width: 800px;
  margin: 0 auto;
  padding: 1.5rem 1rem;
}

header {
  text-align: center;
  margin-bottom: 1.5rem;
}

header h1 {
  font-size: 1.75rem;
  font-weight: 700;
  margin-bottom: 0.25rem;
}

.subtitle {
  color: var(--muted);
  font-size: 0.95rem;
}

/* Tabs */
.tabs {
  display: flex;
  flex-wrap: wrap;
  gap: 0.4rem;
  margin-bottom: 1.5rem;
  justify-content: center;
}

.tab {
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: 6px;
  color: var(--muted);
  padding: 0.5rem 0.9rem;
  font-size: 0.85rem;
  cursor: pointer;
  transition: background 0.15s, color 0.15s, border-color 0.15s;
  font-family: inherit;
}

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

.tab.active {
  background: var(--accent);
  color: #fff;
  border-color: var(--accent);
}

/* Panels */
.calc-panel {
  display: none;
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: 10px;
  padding: 1.5rem;
}

.calc-panel.active {
  display: block;
}

.calc-panel h2 {
  font-size: 1.15rem;
  font-weight: 600;
  margin-bottom: 1.25rem;
}

/* Input rows */
.input-row {
  display: flex;
  align-items: flex-end;
  gap: 0.75rem;
  margin-bottom: 1.25rem;
}

.input-row label {
  flex: 1;
  display: flex;
  flex-direction: column;
  gap: 0.35rem;
}

.input-row label span {
  font-size: 0.82rem;
  color: var(--muted);
  font-weight: 500;
}

.input-separator {
  color: var(--muted);
  font-size: 0.9rem;
  padding-bottom: 0.55rem;
  flex-shrink: 0;
}

input[type="number"],
select {
  background: var(--bg);
  border: 1px solid var(--border);
  border-radius: 6px;
  color: var(--text);
  padding: 0.6rem 0.75rem;
  font-size: 1rem;
  font-family: inherit;
  width: 100%;
  transition: border-color 0.15s;
  -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);
}

select {
  cursor: pointer;
  appearance: none;
  background-image: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='12' height='12' viewBox='0 0 12 12'%3E%3Cpath fill='%239aa0b0' d='M6 8L1 3h10z'/%3E%3C/svg%3E");
  background-repeat: no-repeat;
  background-position: right 0.75rem center;
  padding-right: 2rem;
}

.select-label {
  flex: 0.8;
}

/* Three column layout for add/sub */
.three-col {
  flex-wrap: wrap;
}

/* Result box */
.result-box {
  background: var(--bg);
  border: 1px solid var(--border);
  border-radius: 10px;
  padding: 1rem 1.25rem;
  display: flex;
  align-items: center;
  justify-content: space-between;
  margin-bottom: 0.75rem;
}

.result-label {
  font-size: 0.85rem;
  color: var(--muted);
  font-weight: 500;
}

.result-value {
  font-size: 1.4rem;
  font-weight: 700;
  color: var(--accent);
}

/* Formula box */
.formula-box {
  font-size: 0.82rem;
  color: var(--muted);
  min-height: 1.2rem;
  font-family: 'SF Mono', 'Fira Code', 'Consolas', monospace;
}

/* Footer */
footer {
  text-align: center;
  padding: 2rem 0 1rem;
  color: var(--muted);
  font-size: 0.82rem;
}

/* Responsive */
@media (max-width: 540px) {
  header h1 {
    font-size: 1.4rem;
  }

  .tabs {
    gap: 0.3rem;
  }

  .tab {
    font-size: 0.78rem;
    padding: 0.4rem 0.65rem;
  }

  .calc-panel {
    padding: 1rem;
  }

  .input-row {
    flex-direction: column;
    align-items: stretch;
    gap: 0.6rem;
  }

  .input-separator {
    text-align: center;
    padding: 0;
  }

  .result-value {
    font-size: 1.2rem;
  }
}