To round corners of Gallery Block Carousel, like this.
You can use this code to Custom CSS
[data-block-json*="slider"] {
.slide, img, .content-fill, .content-fit {
border-top-left-radius: 30px;
border-top-right-radius: 30px;
border-bottom-left-radius: 30px;
border-bottom-right-radius: 30px;
}}