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

:root {
  --bg: #0d1117;
  --bg-secondary: #161b22;
  --text: #c9d1d9;
  --text-primary: #f0f6fc;
  --text-muted: #8b949e;
  --border: #21262d;
  --border-input: #30363d;
  --accent: #58a6ff;
  --hover-bg: #161b22;
  --btn-reset-bg: #21262d;
  --btn-reset-border: #30363d;
  --btn-reset-hover: #30363d;
}

.light {
  --bg: #ffffff;
  --bg-secondary: #f6f8fa;
  --text: #1f2328;
  --text-primary: #1f2328;
  --text-muted: #656d76;
  --border: #d0d7de;
  --border-input: #d0d7de;
  --accent: #0969da;
  --hover-bg: #f3f4f6;
  --btn-reset-bg: #f6f8fa;
  --btn-reset-border: #d0d7de;
  --btn-reset-hover: #eaeef2;
}

body {
  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
  background: var(--bg);
  color: var(--text);
  min-height: 100vh;
  display: flex;
  flex-direction: column;
  transition: background 0.2s, color 0.2s;
}

header {
  padding: 20px 30px;
  border-bottom: 1px solid var(--border);
  display: flex;
  align-items: center;
  justify-content: space-between;
}

header h1 {
  font-size: 24px;
  font-weight: 600;
  color: var(--text-primary);
}

.theme-btn {
  width: 40px;
  height: 40px;
  border-radius: 8px;
  border: 1px solid var(--border-input);
  background: var(--btn-reset-bg);
  cursor: pointer;
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 20px;
  line-height: 1;
  transition: background 0.15s;
  color: var(--text);
}

.theme-btn:hover {
  background: var(--btn-reset-hover);
}

main {
  display: flex;
  flex: 1;
}

.sidebar {
  width: 250px;
  border-right: 1px solid var(--border);
  padding: 16px 0;
  flex-shrink: 0;
}

.category-tabs {
  display: flex;
  flex-wrap: wrap;
  gap: 4px;
  padding: 0 12px 8px;
  border-bottom: 1px solid var(--border);
  margin-bottom: 4px;
}

.category-tab {
  padding: 6px 12px;
  border-radius: 6px;
  border: 1px solid var(--border-input);
  background: transparent;
  color: var(--text-muted);
  font-size: 13px;
  cursor: pointer;
  transition: all 0.15s;
  font-family: inherit;
  letter-spacing: 0.3px;
  white-space: nowrap;
}

.category-tab:hover {
  background: var(--hover-bg);
  color: var(--text);
  border-color: var(--text-muted);
}

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

.formula-list {
  list-style: none;
}

.formula-list li {
  padding: 14px 24px;
  cursor: pointer;
  color: var(--text-muted);
  transition: all 0.15s;
  border-left: 3px solid transparent;
  font-size: 16px;
  letter-spacing: 0.5px;
}

.formula-list li:hover {
  background: var(--hover-bg);
  color: var(--text);
}

.formula-list li.active {
  background: var(--hover-bg);
  color: var(--text-primary);
  border-left-color: var(--accent);
  font-weight: 500;
}

.content {
  flex: 1;
  padding: 48px 40px;
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: flex-start;
}

.formula-name {
  font-size: 18px;
  color: var(--accent);
  font-weight: 500;
  letter-spacing: 0.3px;
}

.formula-display {
  font-size: 40px;
  color: var(--text-primary);
  margin: 28px 0 32px;
  padding: 28px 56px;
  background: var(--bg-secondary);
  border-radius: 12px;
  font-family: 'Times New Roman', Times, serif;
  letter-spacing: 1px;
  border: 1px solid var(--border);
  transition: background 0.2s;
}

.constants {
  display: flex;
  flex-wrap: wrap;
  gap: 8px;
  justify-content: center;
  margin-bottom: 16px;
  min-height: 0;
}

.const-item {
  padding: 6px 14px;
  background: var(--bg-secondary);
  border: 1px solid var(--border);
  border-radius: 6px;
  font-size: 14px;
  color: var(--text-muted);
  font-family: 'Times New Roman', Times, serif;
  letter-spacing: 0.5px;
}

.inputs {
  display: flex;
  gap: 20px;
  flex-wrap: wrap;
  justify-content: center;
  margin-bottom: 24px;
}

.input-group {
  display: flex;
  flex-direction: column;
  gap: 6px;
}

.input-group label {
  font-size: 13px;
  color: var(--text-muted);
  text-transform: uppercase;
  letter-spacing: 0.5px;
}

.input-group input {
  padding: 10px 14px;
  background: var(--bg);
  border: 1px solid var(--border-input);
  border-radius: 6px;
  color: var(--text);
  font-size: 18px;
  width: 160px;
  outline: none;
  transition: border-color 0.2s, background 0.2s;
}

.input-group input:focus {
  border-color: var(--accent);
}

.btn {
  padding: 12px 32px;
  border-radius: 6px;
  font-size: 16px;
  cursor: pointer;
  transition: background 0.15s;
  font-weight: 500;
  letter-spacing: 0.3px;
}

.btn-reset {
  background: var(--btn-reset-bg);
  color: var(--text);
  border: 1px solid var(--btn-reset-border);
}

.btn-reset:hover {
  background: var(--btn-reset-hover);
}

.result {
  margin-top: 28px;
  font-size: 28px;
  color: var(--text-primary);
  padding: 16px 36px;
  background: var(--bg-secondary);
  border-radius: 8px;
  min-width: 200px;
  text-align: center;
  display: none;
  border: 1px solid var(--border);
  transition: background 0.2s;
}

.result.visible {
  display: block;
}

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

.result .unit {
  color: var(--text-muted);
  font-size: 18px;
  margin-left: 4px;
}

@media (max-width: 640px) {
  main {
    flex-direction: column;
  }

  header {
    padding: 16px 16px;
  }

  .sidebar {
    width: 100%;
    border-right: none;
    border-bottom: 1px solid var(--border);
    padding: 8px 0;
  }

  .category-tabs {
    flex-wrap: nowrap;
    overflow-x: auto;
    padding: 0 8px 8px;
    border-bottom: 1px solid var(--border);
    margin-bottom: 4px;
    -webkit-overflow-scrolling: touch;
  }

  .category-tab {
    flex-shrink: 0;
  }

  .formula-list {
    display: flex;
    overflow-x: auto;
    padding: 0 12px 8px;
    gap: 4px;
  }

  .formula-list li {
    padding: 10px 16px;
    white-space: nowrap;
    border-left: none;
    border-bottom: 3px solid transparent;
    font-size: 14px;
  }

  .formula-list li.active {
    border-left-color: transparent;
    border-bottom-color: var(--accent);
  }

  .content {
    padding: 24px 16px;
  }

  .formula-display {
    font-size: 28px;
    padding: 20px 24px;
  }

  .input-group input {
    width: 140px;
  }
}