:root {
  --primary-color: #1e90ff;
  --button-bg: #000;
  --button-disabled-bg: #999;
  --background-color: #eaf1f8;
  --card-bg: #fff;
  --text-color: #000;
  --result-color: green;
}

body.dark {
  --background-color: #121212;
  --card-bg: #1f1f1f;
  --text-color: #eee;
  --button-disabled-bg: #666;
  --result-color: #90ee90;
}

body {
  font-family: "Segoe UI", sans-serif;
  background: var(--background-color);
  color: var(--text-color);
  display: flex;
  justify-content: center;
  align-items: center;
  min-height: 100vh;
  padding: 1rem;
  overflow: hidden;
  transition: background 0.3s, color 0.3s;
}

.card {
  position: relative;
  background: var(--card-bg);
  padding: 2rem;
  border-radius: var(--border-radius);
  box-shadow: 0 8px 24px rgba(0, 0, 0, 0.15);
  max-width: 480px;
  width: 100%;
  transition: background 0.3s;
}

#darkModeToggle {
  position: absolute;
  top: 1rem;
  right: 1rem;
  background: transparent;
  border: none;
  font-size: 1.25rem;
  cursor: pointer;
  color: var(--text-color);
}

h2 {
  margin-top: 0;
  font-size: 1.5rem;
  text-align: center;
}

.form-group {
  display: flex;
  flex-direction: column;
  gap: 1rem;
  margin: 1.5rem 0;
}

input[type="number"] {
  width: 20%;
}

input[type="number"],
select {
  flex: 1;
  padding: 0.75rem;
  border: 1px solid #ccc;
  border-radius: var(--border-radius);
  font-size: 1rem;
  transition: border-color 0.2s;
  background-color: #fdfdfd;
}

input:focus,
select:focus {
  outline: none;
  border-color: var(--primary-color);
}

#swapBtn {
  background-color: transparent;
  border: none;
  font-size: 1.25rem;
  cursor: pointer;
  padding: 0.5rem;
  transition: transform 0.3s ease, background-color 0.2s;
}

#swapBtn:hover {
  transform: rotate(180deg);
}

#swapBtn:active {
  transform: rotate(360deg);
}

#swapBtn:disabled {
  opacity: 0.5;
  cursor: not-allowed;
}

#convertBtn {
  padding: 0.75rem 1.5rem;
  background-color: var(--button-bg);
  color: #fff;
  font-size: 1rem;
  border: none;
  border-radius: var(--border-radius);
  cursor: pointer;
  transition: background-color 0.3s;
}

#convertBtn:disabled {
  background-color: var(--button-disabled-bg);
  cursor: not-allowed;
}

.result {
  margin-top: 1rem;
  font-size: 1rem;
  color: var(--result-color);
  text-align: center;
  opacity: 0;
  transform: translateY(10px);
  transition: opacity 0.4s ease, transform 0.4s ease;
}

.result.show {
  opacity: 1;
  transform: translateY(0);
}