How can I change the arrows of the "Slideshow Simple" to be the same as the arrows on the "Slideshow Reel"?

I can't get Reel Arrow Url. So you need to upload an image with style same as Reel. Then replace Pixabay url with your url.

Design > Custom CSS

.gallery-slideshow-controls svg {
    display: none !important;
}

.gallery-slideshow-control.gallery-slideshow-control-prev div, .gallery-slideshow-control.gallery-slideshow-control-next div {
    background-image: url(https://cdn.pixabay.com/photo/2021/12/26/19/27/nature-6895756_640.jpg);
    background-size: contain;
    background-repeat: no-repeat;
    background-position: center center;
    width: 30px;
    height: 30px;
}
.gallery-slideshow-control.gallery-slideshow-control-next div {
    background-image: url(https://cdn.pixabay.com/photo/2021/01/01/15/31/sushi-balls-5878892_1280.jpg);
}