Rounded corners in cover browser?

Hello. I'm not too good with technology.
When using Caliber, and creating a "Content server", I found that book cover images with rounded 4 corners look very beautiful and I really like that.
So in the offline library on the computer, is there any customization to make the covers round like that? If so, please help me with the position to adjust, because I'm still figuring it out little by little.
Thank you very much. Sorry if my expression was confusing.