:root {
  --bg-color: #0f172a;
  --card-bg: rgba(30, 41, 59, 0.7);
  --border-color: rgba(255, 255, 255, 0.1);
  --text-primary: #f8fafc;
  --text-secondary: #94a3b8;
  --accent-color: #3b82f6;
  --accent-hover: #2563eb;
  --bg-blob-1: #3b82f6;
  --bg-blob-2: #8b5cf6;
  --bg-blob-3: #ec4899;
  --input-bg: rgba(15, 23, 42, 0.5);

  --radius-lg: 24px;
  --radius-md: 12px;
  --radius-sm: 8px;
}

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

body {
  font-family: "Outfit", "Noto Sans TC", sans-serif;
  font-size: 20px;
  background-color: var(--bg-color);
  color: var(--text-primary);
  min-height: 100vh;
  display: flex;
  flex-direction: column;
  overflow-x: hidden;
  position: relative;
  -webkit-font-smoothing: antialiased;
}

/* Background Animation */
.background-container {
  position: fixed;
  top: 0;
  left: 0;
  width: 100vw;
  height: 100vh;
  z-index: -1;
  overflow: hidden;
  background: #0f172a;
}

.blob {
  position: absolute;
  filter: blur(80px);
  opacity: 0.6;
  border-radius: 50%;
  animation: float 20s infinite alternate cubic-bezier(0.4, 0, 0.2, 1);
}

.blob-1 {
  width: 400px;
  height: 400px;
  background: var(--bg-blob-1);
  top: -10%;
  left: -10%;
  animation-delay: 0s;
}

.blob-2 {
  width: 350px;
  height: 350px;
  background: var(--bg-blob-2);
  bottom: -10%;
  right: -10%;
  animation-delay: -5s;
}

.blob-3 {
  width: 300px;
  height: 300px;
  background: var(--bg-blob-3);
  top: 40%;
  left: 40%;
  animation-delay: -10s;
}

@keyframes float {
  0% {
    transform: translate(0, 0) scale(1);
  }
  33% {
    transform: translate(30px, -50px) scale(1.1);
  }
  66% {
    transform: translate(-20px, 20px) scale(0.9);
  }
  100% {
    transform: translate(0, 0) scale(1);
  }
}

/* Navbar */
.navbar {
  display: flex;
  justify-content: space-between;
  align-items: center;
  padding: 0.5rem 1.5rem;
  background: rgba(15, 23, 42, 0.3);
  backdrop-filter: blur(10px);
  -webkit-backdrop-filter: blur(10px);
  border-bottom: 1px solid var(--border-color);
  position: sticky;
  top: 0;
  z-index: 100;
}

.nav-brand {
  display: flex;
  align-items: center;
  gap: 0.5rem;
  font-size: 20px;
  font-weight: 700;
}

.logo-icon {
  background: linear-gradient(135deg, var(--bg-blob-1), var(--bg-blob-2));
  -webkit-background-clip: text;
  background-clip: text;
  -webkit-text-fill-color: transparent;
}

.lang-btn {
  background: var(--card-bg);
  border: 1px solid var(--border-color);
  color: var(--text-primary);
  padding: 0.3rem 0.75rem;
  border-radius: var(--radius-sm);
  cursor: pointer;
  font-weight: 600;
  font-size: 20px;
  display: flex;
  align-items: center;
  gap: 0.4rem;
  transition: all 0.3s ease;
}

.lang-btn:hover {
  background: rgba(255, 255, 255, 0.1);
  transform: translateY(-2px);
}

/* Main Layout — single column, category on top */
.main-container {
  display: flex;
  flex-direction: column;
  gap: 0.75rem;
  max-width: 1100px;
  width: 100%;
  margin: 0.75rem auto;
  padding: 0 1rem;
  flex-grow: 1;
}

/* Category Section — top, full width */
.category-section {
  display: flex;
  flex-direction: column;
  gap: 0.4rem;
}

.section-title {
  font-size: 20px;
  font-weight: 600;
  color: var(--text-secondary);
  margin-bottom: 0.2rem;
}

/* 5 columns, compact buttons */
.category-grid {
  display: grid;
  grid-template-columns: repeat(5, 1fr);
  gap: 0.4rem;
}

.category-btn {
  background: var(--card-bg);
  border: 1px solid var(--border-color);
  border-radius: var(--radius-sm);
  padding: 0.45rem 0.3rem;
  color: var(--text-secondary);
  cursor: pointer;
  display: flex;
  flex-direction: row;
  align-items: center;
  justify-content: center;
  gap: 0.4rem;
  transition: all 0.3s cubic-bezier(0.4, 0, 0.2, 1);
  backdrop-filter: blur(10px);
  -webkit-backdrop-filter: blur(10px);
  white-space: nowrap;
  overflow: hidden;
}

.category-btn i {
  font-size: 20px;
  transition: transform 0.3s ease;
  flex-shrink: 0;
}

.category-btn font,
.category-btn span {
  font-size: 20px;
  font-weight: 500;
  overflow: hidden;
  text-overflow: ellipsis;
}

.category-btn:hover {
  background: rgba(255, 255, 255, 0.05);
  border-color: rgba(255, 255, 255, 0.2);
  transform: translateY(-1px);
}

.category-btn.active {
  background: linear-gradient(
    135deg,
    rgba(59, 130, 246, 0.2),
    rgba(139, 92, 246, 0.2)
  );
  border-color: var(--accent-color);
  color: var(--text-primary);
  box-shadow: 0 2px 12px rgba(59, 130, 246, 0.2);
}

.category-btn.active i {
  color: var(--accent-color);
  transform: scale(1.1);
}

/* Converter Section */
.converter-section {
  display: flex;
  justify-content: center;
  align-items: flex-start;
}

.converter-card {
  background: var(--card-bg);
  backdrop-filter: blur(16px);
  -webkit-backdrop-filter: blur(16px);
  border: 1px solid var(--border-color);
  border-radius: var(--radius-lg);
  padding: 1.2rem 1.5rem;
  width: 100%;
  box-shadow: 0 10px 30px rgba(0, 0, 0, 0.3);
}

.converter-header {
  display: flex;
  align-items: center;
  gap: 0.75rem;
  margin-bottom: 1rem;
  padding-bottom: 0.75rem;
  border-bottom: 1px solid rgba(255, 255, 255, 0.05);
}

.current-category-icon {
  width: 38px;
  height: 38px;
  background: linear-gradient(135deg, var(--bg-blob-1), var(--bg-blob-2));
  border-radius: 50%;
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 20px;
  color: white;
  box-shadow: 0 4px 10px rgba(59, 130, 246, 0.3);
  flex-shrink: 0;
}

.converter-header h3 {
  font-size: 20px;
  font-weight: 600;
}

/* From/To on same row */
.converter-body {
  display: flex;
  flex-direction: row;
  align-items: center;
  gap: 0.5rem;
}

.input-group {
  flex: 1;
}

.input-group label {
  display: block;
  font-size: 20px;
  color: var(--text-secondary);
  margin-bottom: 0.25rem;
  font-weight: 500;
}

.input-row {
  display: flex;
  background: var(--input-bg);
  border: 1px solid var(--border-color);
  border-radius: var(--radius-md);
  overflow: hidden;
  transition: all 0.3s ease;
}

.input-row:focus-within {
  border-color: var(--accent-color);
  box-shadow: 0 0 0 2px rgba(59, 130, 246, 0.2);
}

.value-input {
  flex: 1;
  background: transparent;
  border: none;
  color: var(--text-primary);
  font-size: 20px;
  font-family: "Outfit", sans-serif;
  font-weight: 500;
  padding: 0.6rem 0.75rem;
  outline: none;
  min-width: 0;
}

.value-input::placeholder {
  color: rgba(255, 255, 255, 0.2);
}

/* Hide arrow on number input */
.value-input::-webkit-outer-spin-button,
.value-input::-webkit-inner-spin-button {
  -webkit-appearance: none;
  margin: 0;
}
.value-input[type="number"] {
  -moz-appearance: textfield;
  appearance: textfield;
}

.select-wrapper {
  position: relative;
  width: 150px;
  background: rgba(255, 255, 255, 0.05);
  border-left: 1px solid var(--border-color);
}

.unit-select {
  width: 100%;
  height: 100%;
  appearance: none;
  background: transparent;
  border: none;
  color: var(--text-primary);
  font-size: 20px;
  font-family: inherit;
  padding: 0 2rem 0 0.6rem;
  outline: none;
  cursor: pointer;
}

.unit-select option {
  background: #1e293b;
  color: white;
}

.select-icon {
  position: absolute;
  right: 0.5rem;
  top: 50%;
  transform: translateY(-50%);
  pointer-events: none;
  color: var(--text-secondary);
  font-size: 20px;
}

/* Swap button between From and To */
.swap-wrapper {
  display: flex;
  flex-direction: column;
  justify-content: flex-end;
  padding-bottom: 0.2rem;
  flex-shrink: 0;
}

.swap-btn {
  width: 36px;
  height: 36px;
  background: var(--accent-color);
  color: white;
  border: none;
  border-radius: 50%;
  font-size: 20px;
  cursor: pointer;
  box-shadow: 0 3px 8px rgba(59, 130, 246, 0.4);
  transition: all 0.3s cubic-bezier(0.175, 0.885, 0.32, 1.275);
  display: flex;
  align-items: center;
  justify-content: center;
}

.swap-btn:hover {
  background: var(--accent-hover);
  transform: scale(1.1);
}

.swap-btn:active {
  transform: scale(0.95);
}

.formula-display {
  margin-top: 0.75rem;
  padding: 0.5rem 0.75rem;
  background: rgba(255, 255, 255, 0.03);
  border-radius: var(--radius-md);
  text-align: center;
  font-size: 20px;
  color: var(--text-secondary);
  border: 1px dashed rgba(255, 255, 255, 0.1);
}

/* Responsive */
@media (max-width: 900px) {
  .category-grid {
    grid-template-columns: repeat(5, 1fr);
  }
}

@media (max-width: 640px) {
  .navbar {
    padding: 0.4rem 0.75rem;
  }

  .main-container {
    padding: 0 0.5rem;
    margin: 0.5rem auto;
    gap: 0.5rem;
  }

  .converter-card {
    padding: 0.75rem 0.75rem;
  }

  /* On small screens, stack From and To vertically */
  .converter-body {
    flex-direction: column;
    align-items: stretch;
  }

  .swap-wrapper {
    flex-direction: row;
    justify-content: center;
    padding: 0;
  }

  .category-grid {
    grid-template-columns: repeat(3, 1fr);
  }

  .select-wrapper {
    width: 130px;
  }
}

@media (max-width: 400px) {
  .category-grid {
    grid-template-columns: repeat(2, 1fr);
  }
}

/* Background Modal */
.modal-overlay {
  position: fixed;
  top: 0;
  left: 0;
  width: 100vw;
  height: 100vh;
  background: rgba(0, 0, 0, 0.7);
  backdrop-filter: blur(10px);
  -webkit-backdrop-filter: blur(10px);
  z-index: 1000;
  display: flex;
  align-items: center;
  justify-content: center;
  opacity: 0;
  pointer-events: none;
  transition: opacity 0.3s ease;
}
.modal-overlay.active {
  opacity: 1;
  pointer-events: auto;
}
.modal-content {
  background: var(--card-bg);
  border: 1px solid var(--border-color);
  border-radius: var(--radius-lg);
  padding: 1.5rem;
  width: 90%;
  max-width: 800px;
  max-height: 80vh;
  position: relative;
  overflow-y: auto;
  box-shadow: 0 20px 40px rgba(0, 0, 0, 0.5);
}
.modal-close {
  position: absolute;
  top: 0.75rem;
  right: 0.75rem;
  background: transparent;
  border: none;
  color: var(--text-secondary);
  font-size: 20px;
  cursor: pointer;
  transition: color 0.3s;
}
.modal-close:hover {
  color: white;
}

.bg-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(130px, 1fr));
  gap: 0.75rem;
}
.bg-item {
  width: 100%;
  height: 90px;
  border-radius: var(--radius-sm);
  object-fit: cover;
  cursor: pointer;
  border: 2px solid transparent;
  transition:
    transform 0.2s,
    border-color 0.2s;
  background: #0f172a;
}
.bg-item:hover {
  transform: scale(1.05);
  border-color: var(--accent-color);
}

/* Copyright Footer */
#copyright {
  text-align: center;
  padding: 1rem 1rem;
  margin-top: 0.5rem;
  background-color: rgba(30, 41, 59, 0.5);
  border-top: 1px solid var(--border-color);
  backdrop-filter: blur(10px);
  -webkit-backdrop-filter: blur(10px);
  z-index: 10;
  position: relative;
}
#copyright h2 {
  font-size: 20px;
  margin-bottom: 0.25rem;
  color: var(--text-primary);
}
#copyright p {
  font-size: 20px;
  color: var(--text-secondary);
}