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

        body {
            background: #ffffff;
            transition: .3s;
        }

        body.dark {
            background: #111827;
            color: white;
        }

        /* HEADER */

        .topbar {
            height: 70px;
            background: linear-gradient(135deg, #e0e030, #181a03);
            color: white;
            display: flex;
            align-items: center;
            justify-content: space-between;
            padding: 0 15px;
            position: sticky;
            top: 0;
            z-index: 1000;
        }

        .back-btn {
            color: white;
            text-decoration: none;
            font-size: 28px;
            font-weight: 600;
        }

        .topbar h1 {
            font-size: 22px;
            flex: 1;
            margin-left: 15px;
        }

        .theme-btn {
            width: 42px;
            height: 42px;
            border: none;
            border-radius: 50%;
            background: white;
            cursor: pointer;
            font-size: 18px;
        }

        /* TOPIC */

        .topic-box {
            margin: 15px;
            padding: 18px;
            background: white;
            border-radius: 12px;
            box-shadow: 0 2px 8px rgba(0, 0, 0, .08);
        }

        body.dark .topic-box {
            background: #1f2937;
        }

        .topic-box h3 {
            color: #4a0808d8;
            margin-bottom: 8px;
        }

        /* PDF */

        #pdf-container {
            width: 100%;
            padding: 10px;
        }

        canvas {
            width: 100% !important;
            height: auto !important;
            display: block;
            margin: 0 auto 12px;
            box-shadow: 0 2px 10px rgba(0, 0, 0, .1);
            background: white;
        }

        /* BACK BUTTON */

        .bottom-back {
            text-align: center;
            padding: 25px;
        }

        .bottom-btn {
            display: inline-block;
            background: #0D6EFD;
            color: white;
            text-decoration: none;
            padding: 12px 22px;
            border-radius: 10px;
            font-weight: 600;
        }