:root {
  --bg-color: #f0f0f0;
  --text-color: #333;
  --header-color: #555;
  --card-bg: #ffffff;
  --button-bg: #007BFF;
  --button-hover: #0056b3;
  --transition-speed: 0.3s;
}

[data-theme="dark"] {
  --bg-color: #121212;
  --text-color: #e0e0e0;
  --header-color: #bb86fc;
  --card-bg: #1e1e1e;
  --button-bg: #bb86fc;
  --button-hover: #9965f4;
}

body {
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
  display: grid;
  place-content: center;
  min-height: 100vh;
  background-color: var(--bg-color);
  color: var(--text-color);
  transition: background-color var(--transition-speed), color var(--transition-speed);
  margin: 0;
}

.theme-toggle {
    position: fixed;
    top: 20px;
    right: 20px;
}

button.toggle-btn {
    background: var(--button-bg);
    color: white;
    border: none;
    padding: 10px 20px;
    border-radius: 20px;
    cursor: pointer;
    font-weight: bold;
    box-shadow: 0 4px 6px rgba(0,0,0,0.1);
    transition: background-color var(--transition-speed), transform 0.2s;
}

button.toggle-btn:hover {
    background: var(--button-hover);
    transform: translateY(-2px);
}