* {
        margin: 0;
        padding: 0;
        box-sizing: border-box;
        font-family: "Poppins", sans-serif;
      }

      :root {
        --primary-color: #5b6abf;
        --secondary-color: #9c64a6;
        --accent-color: #6a5acd;
        --light-color: #e6e6fa;
        --text-color: #2c3e50;
        --white: #ffffff;
      }

      body {
        background: linear-gradient(
          135deg,
          hsl(187, 94%, 79%),
          #bb96ff,
          #7780ff
        );
        margin: 0;
        padding: 0;
        min-height: 100vh;
        color: var(--text-color);
        background-size: 400% 400%;
        animation: gradientBG 15s ease infinite;
        padding-top: 20px;
      }

      @keyframes gradientBG {
        0% {
          background-position: 0% 50%;
        }
        50% {
          background-position: 100% 50%;
        }
        100% {
          background-position: 0% 50%;
        }
      }
      .header {
        position: fixed;
        top: 0;
        left: 0;
        right: 0;
        background: linear-gradient(
          135deg,
          var(--primary-color),
          var(--secondary-color)
        );
        padding: 0 10%;
        display: flex;
        align-items: center;
        justify-content: space-between;
        z-index: 1000;
        box-shadow: 0 4px 12px rgba(0, 0, 0, 0.1);
        border-bottom: 2px solid #fff;
        transition: all 0.3s ease;
      }

      .header:hover {
        box-shadow: 0 6px 16px rgba(0, 0, 0, 0.15);
      }

      .logo {
        font-size: 28px;
        font-family: "Montserrat", sans-serif;
        font-weight: 700;
        color: var(--white);
        text-shadow: 1px 1px 2px rgba(0, 0, 0, 0.2);
        letter-spacing: 1px;
      }

      .navigation ul {
        list-style: none;
        margin: 0;
        padding: 0;
      }

      .header .navigation ul li {
        float: left;
        position: relative;
      }

      .header .navigation ul li a {
        font-family: "Poppins", sans-serif;
        font-size: 16px;
        font-weight: 500;
        color: var(--white);
        text-decoration: none;
        padding: 22px 20px;
        display: block;
        transition: all 0.3s ease;
      }

      .header .navigation ul li a:hover {
        background-color: rgba(255, 255, 255, 0.15);
        transform: translateY(-2px);
      }

      .header .navigation ul li ul {
        position: absolute;
        right: 0;
        width: 220px;
        background: rgba(255, 255, 255, 0.95);
        border-radius: 0 0 8px 8px;
        box-shadow: 0 8px 16px rgba(0, 0, 0, 0.1);
        display: none;
        overflow: hidden;
      }

      .header .navigation ul li ul li a {
        color: var(--text-color);
        padding: 12px 20px;
        font-size: 14px;
      }

      .header .navigation ul li ul li a:hover {
        background-color: var(--light-color);
        color: var(--primary-color);
      }

      .header .navigation ul li:hover > ul {
        display: initial;
        animation: fadeIn 0.3s ease-out;
      }

      #toggle,
      .header label {
        display: none;
        cursor: pointer;
      }

      .menu {
        width: 28px;
        height: 28px;
        filter: brightness(0) invert(1);
      }

      @media (max-width: 950px) {
        .header label {
          display: initial;
        }

        .header {
          padding: 15px 10%;
        }

        .header .navigation {
          position: absolute;
          top: 100%;
          left: 0;
          right: 0;
          background: var(--primary-color);
          display: none;
          border-radius: 0 0 12px 12px;
        }

        .header .navigation ul li {
          width: 100%;
        }

        .header .navigation ul li a {
          padding: 12px 20px;
        }

        .header .navigation ul li ul {
          position: relative;
          width: 100%;
          left: 0;
          background: rgba(255, 255, 255, 0.1);
        }

        .header .navigation ul li ul li a {
          padding-left: 30px;
        }

        #toggle:checked ~ .navigation {
          display: block;
          animation: slideDown 0.4s ease-out;
        }
      }
      .container {
        max-width: 1200px;
        margin: 0 auto;
        padding: 20px;
      }

      h1 {
        text-align: center;
        padding-top: 50px;
        margin-bottom: 30px;
        color: var(--text-color);
        font-family: "Poppins", sans-serif;
        color: #e6e6fa;
      }

      .flashcards-container {
        display: flex;
        flex-wrap: wrap;
        justify-content: center;
        gap: 25px;
        margin-top: 30px;
      }

      .flashcard {
        width: 220px;
        height: 140px;
        perspective: 1000px;
        cursor: pointer;
      }

      .flashcard-inner {
        position: relative;
        width: 100%;
        height: 100%;
        transition: transform 0.6s;
        transform-style: preserve-3d;
      }

      .flashcard.flipped .flashcard-inner {
        transform: rotateY(180deg);
      }

      .flashcard-front,
      .flashcard-back {
        position: absolute;
        width: 100%;
        height: 100%;
        backface-visibility: hidden;
        border-radius: 12px;
        display: flex;
        align-items: center;
        justify-content: center;
        padding: 20px;
        box-shadow: 0 6px 12px rgba(0, 0, 0, 0.15);
        text-align: center;
      }

      .flashcard-front {
        background: linear-gradient(135deg, #697fff, #8ab9ff);
        color: white;
        font-weight: 600;
        font-size: 20px;
      }

      .flashcard-back {
        background: white;
        color: var(--text-color);
        transform: rotateY(180deg);
        font-size: 18px;
        border: 2px solid var(--accent-color);
      }

      .controls {
        display: flex;
        justify-content: center;
        gap: 15px;
        margin-top: 30px;
      }

      .btn {
        padding: 12px 24px;
        border: none;
        border-radius: 30px;
        background: linear-gradient(
          135deg,
          var(--primary-color),
          var(--accent-color)
        );
        color: white;
        font-weight: 600;
        cursor: pointer;
        transition: all 0.3s ease;
        box-shadow: 0 4px 8px rgba(0, 0, 0, 0.1);
      }

      .btn:hover {
        transform: translateY(-3px);
        box-shadow: 0 8px 16px rgba(0, 0, 0, 0.15);
      }

      .btn:active {
        transform: translateY(1px);
      }

      @media (max-width: 768px) {
        .flashcard {
          width: 160px;
          height: 110px;
        }

        .flashcard-front,
        .flashcard-back {
          font-size: 16px;
          padding: 15px;
        }
      }

      @media (max-width: 480px) {
        .flashcard {
          width: 140px;
          height: 100px;
        }

        .controls {
          flex-direction: column;
          align-items: center;
        }

        .btn {
          width: 80%;
        }
      }