You can adjust the following thumbnail styles
[data-wm-plugin="collection-carousel"] {
--wm-cc-aspect-ratio: 1/1;
--wm-cc-border-radius: 15px;
--wm-cc-border-width: 2px;
--wm-cc-border-color: black;
}To Target the thumbnail use this css
[data-wm-plugin="collection-carousel"] .slide-thumbnail{
/*CSS Here*/
}