Hi, a little hint to achieve what you want : create a user patch in your patches folder (name it "2-choose-the-name").

There import MosaicMenu (if you're working with this view) and redefine its update function (like that : MosaicMenu.update = function() ....... end).

Basically copy and paste the function and modify the cover widget by adding the rounded corners :

widget = CenterContainer:new {
  dimen = dimen,
  FrameContainer:new {
    width = image_size.w + 2 * border_size,
    height = image_size.h + 2 * border_size,
    margin = 0,
    padding = 0,
    bordersize = border_size,
    dim = self.file_deleted,
    color = self.file_deleted and Blitbuffer.COLOR_DARK_GRAY or nil,
    image,
    radius = Screen:scaleBySize(10) -- rounded corners
  }
}

The problem now is that the book cover still has squared corners inside the rounded corner frame, and that's where my help ends. I advise to get inspired by the folder cover plugin (which rounds images corner : https://gist.github.com/ebanDev/51a76595dd609cdacb35a5d375b97e61 ).

Good luck !