:root {
    --bg-color: #f1f5f9;
    --calc-bg: #ffffff;
    --text-color: #1e293b;
    --btn-bg: #e2e8f0;
    --btn-hover: #cbd5e1;
    --accent: #6366f1;
  }
  
  body.dark {
    --bg-color: #1e293b;
    --calc-bg: #334155;
    --text-color: #f1f5f9;
    --btn-bg: #475569;
    --btn-hover: #64748b;
    --accent: #a78bfa;
  }
  
  * {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
  }
  
  body {
    font-family: 'Poppins', sans-serif;
    background-color: var(--bg-color);
    color: var(--text-color);
    display: flex;
    flex-direction: column;
    align-items: center;
    min-height: 100vh;
    padding: 20px;
    transition: all 0.3s ease;
  }
  
  .calculator {
    background-color: var(--calc-bg);
    padding: 20px;
    border-radius: 20px;
    box-shadow: 0 10px 25px rgba(0, 0, 0, 0.1);
    width: 320px;
    max-width: 90%;
  }
  
  .display {
    width: 100%;
    height: 60px;
    background-color: var(--btn-bg);
    color: var(--text-color);
    font-size: 2rem;
    text-align: right;
    padding: 10px;
    border: none;
    border-radius: 10px;
    margin-bottom: 20px;
  }
  
  .buttons {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    gap: 10px;
  }
  
  button {
    font-size: 1.4rem;
    padding: 15px;
    background-color: var(--btn-bg);
    color: var(--text-color);
    border: none;
    border-radius: 12px;
    cursor: pointer;
    transition: background-color 0.2s;
  }
  
  button:hover {
    background-color: var(--btn-hover);
  }
  
  .zero {
    grid-column: span 2;
  }
  
  .theme-toggle {
    position: absolute;
    top: 20px;
    right: 20px;
    display: flex;
    align-items: center;
    gap: 10px;
    font-weight: 600;
  }
  
  /* Toggle Switch */
  .switch {
    position: relative;
    display: inline-block;
    width: 40px;
    height: 22px;
  }
  
  .switch input {
    opacity: 0;
    width: 0;
    height: 0;
  }
  
  .slider {
    position: absolute;
    cursor: pointer;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    background-color: #ccc;
    transition: .4s;
    border-radius: 34px;
  }
  
  .slider:before {
    position: absolute;
    content: "";
    height: 16px;
    width: 16px;
    left: 3px;
    bottom: 3px;
    background-color: white;
    transition: .4s;
    border-radius: 50%;
  }
  
  input:checked + .slider {
    background-color: var(--accent);
  }
  
  input:checked + .slider:before {
    transform: translateX(18px);
  }


  .app-header {
    text-align: center;
    margin-bottom: 30px;
    animation: fadeIn 0.8s ease;
  }
  
  .app-title {
    font-size: 3.5rem;
    font-weight: 700;
    background: linear-gradient(135deg, var(--accent), #8b5cf6);
    -webkit-background-clip: text;
    background-clip: text;
    color: transparent;
    margin-bottom: 5px;
    letter-spacing: -1px;
  }
  
  .app-title-accent {
    font-weight: 800;
    color: var(--accent);
  }
  
  .app-subtitle {
    font-size: 1.1rem;
    color: var(--text-color);
    opacity: 0.8;
    font-weight: 400;
  }
  
  @keyframes fadeIn {
    from { opacity: 0; transform: translateY(-20px); }
    to { opacity: 1; transform: translateY(0); }
  }