/* 1. CSS Variables */
:root {
  --primary: #2c3e50;
  --accent: #3498db;
  --bg: #f4f7f6;
  --white: #ffffff;
  --border: #e0e0e0;
  --error: #e74c3c;
  --success: #27ae60;
  --radius: 8px;
  --transition: 0.3s ease;
}

/* 2. Global Styles */
* {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
  font-family: "Segoe UI", Tahoma, Geneva, Verdana, sans-serif;
}

body {
  background-color: var(--bg);
  color: var(--primary);
  padding: 20px;
  display: flex;
  flex-direction: column;
  align-items: center;
  min-height: 100vh;
}

main {
  width: 100%;
  max-width: 500px;
  background: var(--white);
  padding: 2rem;
  border-radius: var(--radius);
  box-shadow: 0 4px 15px rgba(0, 0, 0, 0.05);
}

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

header h1 {
  font-size: 1.8rem;
  letter-spacing: 1px;
  color: var(--primary);
}

header p {
  font-size: 0.9rem;
  color: #7f8c8d;
  margin-top: 0.3rem;
}

/* 3. Navigation / Tabs */
.tabs {
  display: flex;
  justify-content: space-around;
  gap: 5px;
  margin-bottom: 2rem;
  border-bottom: 2px solid var(--border);
  width: 100%;
}

.tab-link {
  flex: 1;
  padding: 12px 5px;
  font-size: 0.9rem;
  border: none;
  background: none;
  cursor: pointer;
  font-weight: 600;
  color: #95a5a6;
  transition: var(--transition);
  position: relative;
}

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

.tab-link:focus-visible {
  outline: 2px solid var(--accent);
  outline-offset: 2px;
  border-radius: 4px;
}

.tab-link.active {
  color: var(--accent);
  border-bottom: 3px solid var(--accent);
  margin-bottom: -2px;
}

/* 4. Form Elements */
.tab-content {
  display: none;
}

.tab-content.active {
  display: block;
  animation: fadeIn 0.3s ease;
}

@keyframes fadeIn {
  from { opacity: 0; transform: translateY(10px); }
  to { opacity: 1; transform: translateY(0); }
}

.calc-form {
  margin-bottom: 1rem;
}

.input-group {
  margin-bottom: 1.2rem;
}

label {
  display: block;
  font-size: 0.9rem;
  margin-bottom: 6px;
  font-weight: 600;
  color: var(--primary);
}

.row {
  display: flex;
  gap: 10px;
}

input,
select {
  width: 100%;
  padding: 10px 12px;
  border: 2px solid var(--border);
  border-radius: var(--radius);
  font-size: 1rem;
  transition: var(--transition);
}

input:focus,
select:focus {
  border-color: var(--accent);
  outline: none;
  box-shadow: 0 0 0 3px rgba(52, 152, 219, 0.1);
}

button {
  width: 100%;
  padding: 12px;
  background-color: var(--primary);
  color: white;
  border: none;
  border-radius: var(--radius);
  cursor: pointer;
  font-weight: 600;
  font-size: 1rem;
  transition: var(--transition);
}

button:hover {
  background-color: #34495e;
  transform: translateY(-1px);
  box-shadow: 0 4px 8px rgba(0, 0, 0, 0.15);
}

button:active {
  transform: translateY(0);
}

button:focus-visible {
  outline: 3px solid var(--accent);
  outline-offset: 2px;
}

/* 5. Results Area */
.result,
.result-mini {
  margin-top: 1.5rem;
  padding: 15px;
  background: #e8f4fd;
  border-radius: var(--radius);
  text-align: center;
  font-weight: 600;
  border-left: 4px solid var(--accent);
}

.result-mini {
  margin-top: 10px;
}

.output,
.output-s,
.output-w,
.output-v1,
.output-water {
  font-family: "Courier New", Courier, monospace;
  font-size: 1.2rem;
  color: var(--accent);
  font-weight: bold;
}

.copy-btn {
  width: auto;
  background: transparent;
  color: var(--accent);
  font-size: 1.1rem;
  padding: 4px 10px;
  margin-left: 10px;
  box-shadow: none;
}

.copy-btn:hover {
  background: rgba(52, 152, 219, 0.1);
  transform: scale(1.1);
}

.copy-btn:focus-visible {
  outline: 2px solid var(--accent);
}

.copy-btn.success {
  color: var(--success);
}

/* 6. Conversion Cards */
.conv-card {
  background: #f9f9f9;
  padding: 15px;
  border-radius: var(--radius);
  margin-bottom: 20px;
  border-left: 4px solid var(--accent);
}

.conv-card h4 {
  font-size: 0.95rem;
  margin-bottom: 12px;
  color: var(--primary);
}

/* 7. Error Toast */
.error-toast {
  position: fixed;
  top: 20px;
  left: 50%;
  transform: translateX(-50%);
  background: var(--error);
  color: white;
  padding: 12px 24px;
  border-radius: var(--radius);
  box-shadow: 0 4px 12px rgba(231, 76, 60, 0.3);
  font-weight: 600;
  z-index: 1000;
  animation: slideDown 0.3s ease;
}

.error-toast.hidden {
  display: none;
}

@keyframes slideDown {
  from { transform: translate(-50%, -100%); opacity: 0; }
  to { transform: translate(-50%, 0); opacity: 1; }
}

/* 8. Responsive Design */
@media (max-width: 600px) {
  body {
    padding: 10px;
  }

  main {
    padding: 1.5rem;
  }

  header h1 {
    font-size: 1.5rem;
  }

  .tabs {
    gap: 3px;
  }

  .tab-link {
    font-size: 0.8rem;
    padding: 10px 3px;
  }

  input,
  select,
  button {
    font-size: 0.95rem;
  }

  .output,
  .output-s,
  .output-w,
  .output-v1,
  .output-water {
    font-size: 1rem;
  }

  .result,
  .result-mini {
    font-size: 0.9rem;
    padding: 12px;
  }
}

@media (max-width: 400px) {
  .tabs {
    flex-wrap: wrap;
  }

  .tab-link {
    flex: 1 1 45%;
    padding: 10px;
  }
}

/* 9. Print Styles */
@media print {
  body {
    background: white;
  }

  .tabs,
  button {
    display: none;
  }

  .result {
    border: 1px solid #000;
  }
}