book-header.css

@keyframes shiftGradient {
    0% {
        background-position: 0% 50%;
    }

    100% {
        background-position: 100% 50%;
    }
}

@keyframes shiftGradientBack {
    0% {
        background-position: 100% 50%;
    }

    100% {
        background-position: 0% 50%;
    }
}

header {
    background-color: var(--bg);
    position: relative;
    border-radius: 60% 0 0 0;
    padding-top: 4rem;
}

header::after {
    --accent: #98E6B5ff;
    --shadow: #0000003b;
}

@media (prefers-color-scheme: dark) {
    header::after {
        --accent: #98E6B5ff;
        --shadow: #ffffff3b;
    }
}

@media (min-width: 600px) {
    header {
        padding-top: 6rem !important;
    }

    header::after {
        content: '';
        position: absolute;
        width: 100%;
        /* Match the width of the h1 */
        height: 100%;
        /* Match the height of the h1 */
        left: 1.6rem;
        /* Horizontal position of the shadow */
        top: -2.1rem;
        /* Vertical position of the shadow */
        background:
            /* Gradient from shadow color to accent color */
            linear-gradient(to right, var(--shadow) 50%, var(--accent));
        background-size: 250% 100%;
        /* Make the gradient larger than the element */
        filter: blur(8px);
        /* Blur effect for the shadow */
        z-index: -1;
        /* Ensure the shadow is behind the text */
        border-radius: 60% 0 0 0;
        /* Match the border-radius of the h1 */
        transition: all 0.3s ease-in-out;
        animation: shiftGradientBack 0.8s ease forwards;
    }

    header:hover::after {
        animation: shiftGradient 0.8s ease forwards;
    }
}