Update code

/* change to hand on hover previous next */
.gallery-slideshow-control-prev .gallery-slideshow-control-btn:before, .gallery-slideshow-control-next .gallery-slideshow-control-btn:after, .gallery-fullscreen-slideshow-control-btn::before, .gallery-slideshow-control-btn::before {
    cursor: pointer;
}
/* grey to black */
/* background color */
button.gallery-slideshow-control-btn:before {
    background: transparent !important;
}
@media screen and (max-width:767px) {
   .gallery-slideshow[data-controls-location="simple"] .gallery-slideshow-control-btn[data-previous] {
       margin-left: 0px !important;
       padding-left: 0px !important;
   } 
     .gallery-slideshow[data-controls-location="simple"] .gallery-slideshow-control-btn[data-next] {
       margin-right: 0px !important;
        padding-right: 0px !important;
   } 
}