Add this code to the Custom CSS field on the Appearance tab of your YouTube Gallery widget’s settings:
.yottie-widget-feed-section-arrow {
scale: 1.5 !important;
}
.yottie-widget-feed-section-arrow-prev {
top: 10px !important;
}
.yottie-widget-feed-section-arrow-next {
bottom: 10px !important;
}