body {
    margin: 0;
    font-family: Arial, sans-serif;
  }
  
  header {
    background-color: #333;
    color: #fff;
    padding: 20px;
  }
  
  nav {
    background-color: #444;
    display: flex;
    justify-content: space-between;
    align-items: center;
    padding: 10px 20px;
  }
  
  nav ul {
    list-style-type: none;
    margin: 0;
    padding: 0;
    display: flex;
  }
  
  nav li {
    margin-right: 20px;
  }
  
  nav a {
    color: #fff;
    text-decoration: none;
  }
  
  .main-content {
    display: grid;
    grid-template-columns: repeat(3, 1fr);
    grid-gap: 20px;
    padding: 20px;
  }
  
  .box {
    background-color: #f1f1f1;
    padding: 20px;
    text-align: center;
    position: relative;
    overflow: hidden;
  }
  
  .box::before {
    content: "";
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    border: 4px solid transparent;
    border-image: linear-gradient(to right, #4CAF50, #2196F3) 1;
    animation: border-animation 5s linear infinite;
    z-index: 1;
  }
  
  .box img {
    max-width: 100%;
    height: auto;
    display: block;
  }
  
  footer {
    background-color: #333;
    color: #fff;
    padding: 10px;
    position: fixed;
    left: 0;
    bottom: 0;
    width: 100%;
    text-align: center;
  }
  
  @keyframes border-animation {
    0% {
      border-image-slice: 1;
    }
    50% {
      border-image-slice: 100;
    }
    100% {
      border-image-slice: 1;
    }
  }
  
  @media (max-width: 768px) {
    nav {
      flex-direction: column;
      align-items: flex-start;
    }
  
    nav ul {
      flex-direction: column;
      margin-top: 10px;
    }
  
    nav li {
      margin-right: 0;
      margin-bottom: 10px;
    }
  
    .main-content {
      grid-template-columns: 1fr;
    }
  }