How to change Slideshow Reel arrows background

You can use this code to Custom CSS box

/* Reel arrows transparent */
.gallery-reel-control-btn::before {
background-color: #f1f !important;
}

If you want to make it transparent, use this CSS code

/* Reel arrows transparent */
.gallery-reel-control-btn::before {
background-color: transparent !important;
}