body {
      background-color: #f0f0f0; 
      display: flex;
      justify-content: center;
      align-items: center;
      height: 100vh;
      margin: 0;
    }

    .calculator {
      background-color: #cbd4ce; 
      border-radius: 20px;
      padding: 20px;
      box-shadow: 0 10px 20px rgba(0, 0, 0, 0.1); 
      display: grid;
      grid-template-columns: repeat(4, 80px);
      grid-gap: 10px;
      border: 2px solid #444; 
      position: relative; 
    }

    .display {
      grid-column: 1 / -1;
      background-color: #666;
      color: #fff;
      padding: 10px;
      font-size: 24px;
      text-align: right;
      border-radius: 10px;
      margin-bottom: 10px;
    }

    .button {
      width: 60px;
      height: 60px;
      border-radius: 50%;
      border: none;
      font-size: 18px;
      background-color: #666;
      color: #fff;
      cursor: pointer;
      transition: background-color 0.3s, transform 0.3s;
      transform-style: preserve-3d;
    }

    .button:hover {
      background-color: #888;
      transform: translateY(4px);
      box-shadow: 0 2px 5px rgba(0, 0, 0, 0.3);
    }

    .history {
      display: none; 
      position: absolute;
      top: 85px; 
      left: 20px;
      right: 20px;
      bottom: 20px; 
      background-color: #cbd4ce; 
      border: 2px solid #444;
      border-radius: 10px;
      padding: 15px;
      overflow-y: auto;
      z-index: 1000;
      box-shadow: 0 8px 16px rgba(0,0,0,0.4);
    }

    .history.active {
      display: block; 
    }

    #history-list div {
      padding: 8px 0;
      border-bottom: 1px solid #999;
      color: #333;
      font-family: sans-serif;
      font-size: 15px;
    }

    .overlay {
      position: fixed;
      top: 0;
      left: 0;
      width: 100%;
      height: 100%;
      background-color: rgba(0, 0, 0, 0.7);
      display: flex;
      justify-content: center;
      align-items: center;
      z-index: 9999; 
    }

    .brand-name {
      font-size: 32px;
      margin-bottom: 20px;
    }

    @media only screen and (max-width: 768px) {
      .calculator { grid-template-columns: repeat(4, 60px); }
      .button { width: 60px; height: 60px; font-size: 18px; }
      .display { font-size: 20px; }
    }

    @media only screen and (max-width: 480px) {
      .calculator { grid-template-columns: repeat(4, 40px); }
      .button { width: 40px; height: 40px; font-size: 16px; }
      .display { font-size: 18px; }
    }

    .splash-content {
      text-align: center;
      color: white;
      font-size: 24px;
      max-width: 80%;
      margin: auto;
    }

    .splash-content img {
      max-width: 100%;
      height: auto;
      margin-bottom: 20px;
    }

    @media only screen and (max-width: 768px) {
      .splash-content { font-size: 20px; }
    }

    @media only screen and (max-width: 480px) {
      .splash-content { font-size: 18px; }
    }